Alco C-855 R-T-R Build – Part 2 – Grab Irons

This week I’ll be continuing my step-by-step build of an N Scale A-B-A Ready-To-Run set of Alco C-855 locomotives.  And this post will be about the first details to be applied to the shells; the grab irons.  You can find part one of the build here.

The grab irons or hand rails are small parts but form an important detail.  For these models I’ve made them from etched brass rather than a part of the 3D printed shell.  I could make them a part of the shell but they would be extremely fragile and probably couldn’t withstand being handled without breaking.  The other option’s to make them a solid piece of the shell but I find that makes them look too bulky.

The grab irons are located in the etched brass frets as shown below.  There are two sets for the A units and one for the B unit.

Each 3D printed shell already has the holes to locate all of the grab irons, and other parts.  Below you can see the cab of the A unit with the various holes.

And the rear also has holes for the four grab irons which create the ladder to the top of the locomotive.  Both ends of the B unit are the same as the rear of the A.

There are two types of grab iron.  Straight and folded down and in the A unit fret there are eight of each, although you only need 7 of the straight ones.

The straight ones have half etched sections where they are connected to the main fret to allow them to be easily cut out with a sharp knife.

One thing I strongly recommend is to test fit each grab iron.  If the hole is clogged or the grab iron is slightly bent the wrong way and you attempt to glue it right in, one leg will stick and the other will bend and you’ll be left with a wonky grab iron.  Each grab iron fits into a pair of holes which are either all the way through the shell or just the right length. So if you’ve cut them out too close to the fret and they are too long they may stick out too far.  This is another good reason to do a test fit.

The folded down version also has half etched sections on the rear just after the corner.   This allows the grab iron to easily be folded down in the right place.

I find by using a pair of wide tweezers I can hold both the legs and simply bend the fold down section into place with my finger.  When I tried it the other way round it was hard to get both legs in the right place.

I tend to get all the grab irons ready together, but I keep the two types separated.

To secure them in place I use superglue.  This is a great choice, not only because it sets very quickly but it is a type of acrylic and so are the 3D printed shells so there’s no danger of a chemical reaction damaging the shell.  I wouldn’t recommend trying to apply superglue directly to the shell; that normally ends in a sticky mess.  The best way is to pour some onto an old box lid or something similar, then gently dip the tips of the grab iron into the superglue just before you place it into the holes.  After you have test fitted it of course.

The straight grab irons fit in the A unit cab in six locations; I know there are only five shown below, I forgot one but it will appear shortly.  There are two in the roof above the number boards, two in the face of the cab above the outer windows, one in the side of the nose, above the step area, and finally, although not shown yet, one on the top of the nose.

The seventh fits in the top of the shell at the rear. The last one is simply a spare.

Three of the drop down grab irons fit into the side of the nose under the straight one.  These are the only three which don’t fit into two holes; the rear leg of each grab iron does but the front simply glues onto the front of the nose.  There is small sections of the 3D printed shell which stick out to locate the grab irons which sit on top of them. There is a fourth fold down grab iron under one of the windows.

When complete the fronts look like this. The B unit front is the same as the rear.

And the rears look like this.

The B unit fret has eight fold down grab irons and two straight ones.

So why have I only fixed the grab irons and not the rest of the etched brass parts?  Well these are the most delicate to do and also these need to blend in when the locomotives are painted. All the rest will be fixed after painting as they either fixed to several different parts or will make painting the shell harder to do if fitted first.

These shells will now go in for painting and while that’s happening I’ll turn my attention to the chassis which I’ll share with you next week.

Alco C-855 R-T-R Build – Part 1 – All the Parts

As promised in an earlier post I’m going to share with you the process of building an N Scale A-B-A Ready-To-Run set of Alco C-855 locomotives.  And this post is the first one covering all the parts.

I released the C-855 kit back in the beginning of 2016 and have made a few since then but as a fellow modeller has asked me to make a complete set for him, two A units and a B, I decided to document the whole thing to help others, as this build is a bit more tricky than normal.

So where to start?

I guess the best place is to show you all the parts you will need.  This will include 3D printed parts, donor parts, new parts and etched parts.

The 3D printed parts, as shown below, are all supplied by Shapeways in their Fine Detail Plastic material; originally called Frosted Detail Plastic.  This material is available in two quality levels; smooth and smoothest. The difference is the layer thickness, Smooth being 29 microns and Smoothest being 16 microns.  The Smoothest option takes longer to print and is therefore more expensive.  Since releasing these models Shapeways have also introduced their option to set the orientation of prints so the best detail can be achieved in the areas where you want it.  However, this also comes at a higher cost and as these engines are so big it did make a considerable difference to the price.  So I offer them in orientated and unorientated versions.  To find out more please see the C-855 page here and the C-855B page here.

The parts 3D printed in Fine Detail Plastic, starting from the top, are:

  • C-855 Locomotive Shell
  • C-855B Locomotive Shell
  • C-855 Locomotive Shell
  • 3 Fuel Tanks & 6 Drive Shaft Extenders
  • 18 Sand Boxes
  • 4 Special Sand Boxes, 4 Crew, 3 Sets Of Horns, 4 More Sand Boxes & 3 Fuel Tank Mounts

Since arriving from Shapeways all the parts above have simply been rinsed under warm water, soaked for twenty-four hours in Goo Gone, rinsed again in warm water, left to dry for forty-eight hours and finally run over lightly with a brush in a Dremel style tool as shown below.

I use this tool as any residue left over from the print process turns to powder after contact with the Goo Gone; once dry the brush simply knocks it off.

The next set of parts is the chassis.  For these locomotives, if you want them to be powered, you will need to get a donor chassis from a Con Cor U50 or turbine.  This is the only thing currently available which is even close to the C-855 chassis.  However it is too short and needs to be lengthened.  I will cover that later.

The parts for the chassis, starting from the top, are:

  • 3 Con Cor U50/Turbine Donor Chassis
  • 3 Sets of 3D Printed Stainless Steel Chassis Extenders
  • 3 New Kato Motors

The chassis extenders are also 3D printed by Shapeways and can also be found on the C-855 page here and the C-855B page here.

The new motors are not necessary for the build but the old Con Cor motor, although reliable and strong, is rather noisy by modern standards and this particular Kato motor works well as a replacement.  You can read a post about swapping them here with and an update here.

The last set of parts, well almost, is the etched brass parts as shown below. These are etched in 12 thou brass to give strength to the long parts.

Each etched set of C-855 Additions, as shown below contains:

  • 7 Handrails
  • 16 Grab Irons
  • 4 Ladders
  • 3 Walkway Platforms
  • 2 Sun Visors
  • 4 Windscreen Wipers
  • 4 MU (Multiple Unit) Hoses
  • 2 Miscellaneous Pipe Sections

The etched set of C-855B Additions, as shown below contains:

  • 8 Handrails
  • 10 Grab Irons
  • 4 Ladders
  • 3 Walkway Platforms
  • 4 MU (Multiple Unit) Hoses
  • 2 Miscellaneous pipe sections

The only other thing I’ll need for this build is three DCC decoders, LEDs for headlights and relevant wire but I’ll come to that in a later post.  I’ll also start working on the preparation of the parts leading up to the assembling of the locomotives.

This week I’ll finish off by saying I, along with my club members and club layout ‘Solent Summit’, will be at the Great Central Railways model railway exhibition from the 15th to the 17th June 2018.  You can find our more here.  And if you time it right you’ll see an A-B-A set of C-855s running on the layout.

3D Print Orientation and What To Do When It’s Wrong

As promised in last week’s post this week I’m going to share with you how to identify if your 3D printed model has been printed correctly.

So what do I mean by correctly printed? Back in October of 2017, in a post which you can find here, I shared with you the new feature from Shapeways which allows the orientation of the print to be set.  This means parts such as a locomotive shell can be printed with the roof on top ensuring the smoothest detail, rather than upside down like a bath tub.

However sometimes, even though the print orientation has been set, some models slip through the printer’s checks and get printed in a cost-saving way; this normally means upside-down.  But how can you tell?  Well, there are a few tell-tale signs which are caused by the print process which give away the orientation of the print.  These signs can be seen when the model is first delivered but given the transparent nature of the material it is fairly hard to spot and nearly impossible to photograph.

So the first thing I always do with any model is soak them in Goo Gone for 24 hours, which makes them opaque, rinse them under warm water and leave to dry for another 24 hours.  Below you can see a set of Alco C-855 shells which have been through this process.  These shells were ordered with the print orientation set so they printed the right way up and at first glance they look good.

But a closer inspection reveals they have been printed upside-down.

The first clue is the direction of the print shadow. The print shadow is the area under a section which sticks out.  In order to print this section support material is required to literally support it. However, where this support material comes into contact with the actual model it leaves a slighty rougher finish which is called the print shadow.  For example, in the image below you can see the print shadow running up from the bolt detail around the base, which means the model was printed upside-down. As the bolt detail protrudes out from the base a bit of support material was required under it. Also looking at the doors and vents on the side of the body you can see these were also covered in support material in order to print the base which also projects out further.

This effect is repeated on the rear as shown below.

The second clue is the inside of the model.  In the picture below you can see all the detail is crisp and smooth.  This is because it hasn’t come into any contact with support material.  This is the best finish on the model and sadly it’s the one location where it’s not needed.

The third clue is the actual top of the model.  It should be smooth, like the inside, but as you can see it’s rougher and ‘furry’ with support material residue which has turned into powder because of the Goo Gone.  The whole of the top of the model has been submerged in support material, because the model was printed upside-down instead of the right way up as requested in the orientation setting.

Now, these shells are not bad and the powder residue can easily be removed with a soft brush in a Dremel style tool, or by hand with a brush, leaving you with a good model.  But the surfaces which should have been on top will never be as good as the finish on the inside and areas such as the doors and vents will also be a bit rougher.

So what should it look like? Below is another set of Alco C-855 shells. You can see that after the cleaning process the finish on the outside is not all the same colour. This is because a lot of the surface hasn’t come into contact with support material, as we wanted.

There is still a print shadow effect but this time it’s running down the model and not up.

The doors and vents still have some print shadow but only in a few areas such as the recess for door hinges etc.

The inside of the shell is rougher and covered in print shadow, as we would expect as it was full of support material.

The top is smooth and very well detailed which will show up when the shells are painted.  In the pictures they look rough or lined but this is simply where the Goo Gone has not affected any support material residue and the surface is still a bit transparent.

Hopefully this will help you identify if a model has been printed in the correct orientation or not.  But what should you do if yours arrives and you think it was printed the wrong way up?

Firstly check to make sure the model was designed to have the orientation set. I can’t speak for other designs but my models will state this in the description if it has been set and I can always confirm if you want to contact me and check.  As for the Alco C-855 shells you need to purchase the Deluxe version as it’s not set on the standard.

Secondly, take some pictures of the incorrect model showing things like the print shadow running the wrong way.  Then send an email to Shapeways at service@shapeways.com.  Include your order number, photos and let them know the model you received has not been printed in the correct orientation. Please note: this must be done within ten days of receiving the model.  Their customer service team are quick to respond and will organize a re-print of the model if indeed it was printed wrongly. But again, you only have ten days from the time you receive your print.

As I said before any excess powder will need to be cleaned off and you will find the detail is good underneath it.  You also need to clean this off otherwise any paint applied will flake off as the powder is loose.

You may also be wondering what I’m doing with so many Alco C-855 shells?  These are for a fellow modeller and I’m making a fully powered ready to run A-B-A set for them.  And I intend to share the whole build process with you in a set of posts which should be starting very soon.

Installing LokSound Select Direct Micro DCC Decoders in Kato Locomotives

This week’s post is a guest post; I had one of these before from fellow N Scale modeller Mike Musick who wrote an article about improving Con-Cors N Scale U50s, Turbines, and my C-855 by replacing the wheel sets.  This time the article has been written by N Scale modeller Chris Hatt who has written about installing ESU LokSound Select Direct Micro DCC decoders into N scale Kato Locomotives.

So without further ado, I’ll hand you over to Chris.

LokSound Select Direct Micro DCC (part number 73100)

So, ESU recently released to market three new decoders designed to fit in N-scale “Narrow Hood” locomotives. These are locomotives such as the EMD “SD” locomotives (SD40/50/6070/80 and 90 series) and the GE Evolution (ES44 series), AC4400 and Dash-9 type models which have an external walkway down each side rather than a full-width body shell. The body shells on these models are typically around 10mm wide inside.

The LokPilot V4 Direct Micro OEM (#54650) and LokSound Select Direct Micro OEM (#73199) are both designed to partner recent locomotives from InterMountain and Atlas and are available factory fitted or aftermarket to retrofit DC models. The one of interest to me is the LokSound Select Direct Micro (#73100). This is designed to “drop into many pre-2016 Atlas and InterMountain locos, (and others with minor modification)” (http://www.esu.eu/en/products/loksound/loksound-select-direct-micro/). Many people have been asking on-line if they will fit into Kato N-scale models and there have been few answers. As most of my locomotives are Kato and I favour using LokSound decoders to install sound, I decided to find out.

So what do you get?  In the blister pack is the decoder, two 3mm golden white LEDs and two lengths of fine brown-insulated wire for connecting a speaker of your choice. The card backing of the blister pack is a fold-out instruction sheet (the LEDs and wire are between the halves of the card in a zip-lock bag, not in the blister).

Figure 1: The 73100 from the top. The front-end is to the left.

You will note that there are five pairs of metal pads along the edges of the 73100. The two pairs nearest each end of the decoder are frame power pickups, red to the top of the photograph and black below. The pair nearest the center on the narrowest part of the decoder are not labelled on any documentation but careful investigation with a continuity tester showed that these are duplicates of the motor power pads on the underside of the decoder. The pair of pads inboard of the seconds power pick ups from the right are the speaker connections. The tiny yellowish rectangle on the centre-line at the left end is a surface mount 0402 LED connected to output AUX1. This LED is around 1.0mm x 0.5mm!

Figure 2: The underside of the 73100. The front-end is to the left.

The two big pads under the decoder are the motor drive outputs. The front-most is the “orange” output touching the right-hand side of the decoder. Near the back of the decoder are a +ve supply (DCC “blue”) pad and pads for the AUX3 and AUX4 function outputs. At each end of the decoder are a pair of pads spaced for soldering the LEDs for the head- and tail-lights (F0F and F0R). According to the instructions, there are current limiting resistors installed on all the outputs and standard LEDs can be soldered directly to the pads. The supplied 3mm LEDs are not attached so that you can cut the leads to the right lengths to position them appropriately for the model that you are installing the decoder in.
The tiny yellowish rectangle on the centre-line at the right-hand end is another surface mount 0402 LED connected as AUX2. It will be very difficult to desolder the AUX1 and AUX2 LEDs and reuse the pads, so while this is technically a six function decoder, two of them will be nigh on impossible to exploit unless it is possible to pipe the light using optical fibre.

How does it compare to a Kato lighting PCB?

Figure 3 shows the 73100 alongside the lighting and power PCBs from several Kato models.

From top to bottom:
• The PCB from an SD80MAC (also used in the SD9043MAC).
• The 73100.
• A PCB from an SD70MAC (also used in the early-SD70M, ES44AC, AC4400CW, and several others) .
• The revised PCB used in the “screwless” later-SD70M and the SD70Ace. This board has sideways-facing surface-mounted LEDs in place of the 3mm discretes on the SD70MAC board.

All four have the front-end of the board to the left.  Putting the decoder in my micrometer, it measures 0.75mm thick compared to the 0.5mm of the Kato PCB.

Fitting the 73100 in a Kato early-SD70M frame.

The 73100 is closest to the early SD70M/SD70MAC/ES44AC/AC4400CW part so I started there.

Figure 4: The 73100 offered up to a Kato SD70M frame.

Offering the 73100 up to the frame, it becomes obvious that the increase in width of the board at the rearmost but one pair of power pickups means that the decoder will not fit between the frame halves without easing back the blocks indicated in figure 5 below.

Figure 5: Easing the fit of the waist of the decoder.

Shaving off about 0.5mm from each side with a file, Dremel or milling machine ensures clearance. It does not matter if the fit is snug enough that the pads touch the frame because the exposed pads are frame power pickups.

Figure 6: This nub needs to be made smaller.

The slightly thicker board of the 73100 means that the rounded end of the nub shown in figure 6 that presses on the contact pad at the front of the decoder needs to be trimmed slightly. While the decoder will not drop-and-slide-in like the PCB, it can be trapped between the frame halves as they are assembled and it make good contact and is firmly fixed fore-and-aft.

However, powering up the decoder in the frame caused it to go into a rapid short-circuit/shut-down cycle as shown by blinking of the AUX1 LED. Oops!

Careful inspection showed that there were a number of surface-mounted components that could foul the frame halves and pass track power into the decoder by unwanted routes.

Figure 7: Easing the frame around the front of the decoder from above.

Figure 8: Easing the frame around the front of the decoder from inside the frame. Note the trimmed nub on the right.

Carefully trimming back the frames as shown in figures 7 and 8 removes this contact and everything works nicely. Note that the trim is above and below where the decoder will sit to clear components on both faces of the PCB.

Figure 9: Test fitting the decoder.

As you can see, there is a gap under the decoder at the back into which a speaker could fit, but I prefer an alternate location as shown later.

Adding LEDs and the motor connections

Next, head and tail-light LEDs are soldered to the undersides of the decoder. I think that the supplied LEDs are a bit too “golden yellow” for a modern locomotive so substituted “clear white” ones:

Figure 10: Supplied (left) and replacement (right) LEDs.

Figure 11: Head and tail-light LEDs fitted, AUX3 ,AUX4 and “blue” wires attached and motor feeds in place. The headlight is on but dimmed under “Rule 17”.

I have fitted green (AUX3), purple (AUX4) and blue (+ve supply) wires to the underside of the decoder in preparation for fitting separately controlled ditch lights later. I provided feeds from the decoder to the motor brushes by using strips of phosphor-bronze 1/16th of an inch wide and 5 thousands of an inch thick (1.6mm x 0.12mm) soldered to the appropriate pads on the decoder. These are pressed against the motor brush tabs by the body shell very much like the connections of the original lighting PCB. To prevent these from contacting the frame-halves, yellow “Kapton” tape has been wrapped around the frame rails under their path. In addition, I placed a strip of Kapton tape under the headlight and under the rear of the decoder to ensure that nothing touched the frame there. This is particularly important at the back as the solder joints attaching the purple, green and blue wires would otherwise rest on the frame.

And, of course, a speaker

My preferred location for the speaker is at the back of the frame. By trimming off the shaded area in figure 12, space is made for an 8mm x 12mm “sugar cube” type speaker (although I buy mobile phone spare parts on eBay rather than commercial “railway modelling” speakers).

Figure 12: The bit of the frame I remove to make room for a speaker.
A suitable baffle can be constructed from plastic sheet, purchased commercially or 3D printed (James does some). I attach the speaker baffle to the end of the frame with an adhesive “sticky dot as in figure 13.

Figure 13: The speaker installed.

The baffle provides most of the insulation needed to keep the speaker from contacting the frame but a short length of Kapton tape on the shelf underneath adds to the protection.

And that’s it, bar loading a suitable sound project and configuring the decoder:

Figure 14: A short video of the installation using the “Drive Hold” feature of the decoder to stop it moving while changing the throttle setting. Still got the ditch lights to do!

That is certainly easier than milling out the fuel tank to take a LokSound Micro V4 or LokSound Select Micro and also leaves the locomotive somewhat heavier as less metal is removed:
• With a Digitrax DN163K1C non sound decoder 116g
• With an ESU LokSound Select Direct Micro and speaker 114g
• With an ESU LokSound Micro V4 and speaker 105g
and weight equals tractive effort.

Figure 15: The same kind of frame with a pocket milled in the fuel tank to take a LokSound Micro V4 (or Select Micro), with channels through the back of the fuel tank, across the bottom of the frame and up the sides to get the wires to the lighting PCB to hook the decoder up.

Where next?

Next, the SD80MAC/SD9043MAC and the late-SD70M/SD709Ace.

I leave you this week by saying thanks to Chris for his post and I look forward to his how-tos on fitting LokSound Select Direct decoders into other locomotives.

Improving Kato UniTrack HO Points for DCC Operation

Kato UniTrack is a very good product and allows reliable trackwork to be assembled quickly without the need to cut and solder track.  Most Kato turnouts, including N scale, have the ability to be switched between power routing and non-power routing, but the No.4 HO turnout, as pictured below, doesn’t. So in this week’s post I’ll show you how I modify Kato UniTrack No.4 turnouts for use with DCC.

But what does power routing mean?  Below is an extract from www.dccwiki.com showing how the turnout isolates different routes depending on how it’s set.

For DC operation, power routing is very useful as power is delivered only where you want the train to run.  The other route is isolated so any trains on that line won’t move.  However for DCC all the tracks want to be powered so the turnout ideally wants to be non-power routing.  As I said earlier most Kato turnouts can be switched between power routing and non-power routing but the HO No.4 can’t.

In the No.4 box you get the actual turnout and associated track parts.

The actual turnout has an all metal frog shown in green, electrically linked blades shown in yellow and switched rails shown in blue.  The stock rails are marked red and black; these have the incoming power.

Between the frog and the switched rails is a plastic insulator.  It’s these two rails which ideally need to be electrically connected permanently for DCC operation.  However the frog changes polarity depending on how the turnout is set so you simply can’t solder the switched rails to the frog.

On the underside of the turnout are five screws holding on the base plate.

Under the base plate you can see the electronic switch and the solenoid which changes the turnout.  In the image below the turnout is set for the straight route. The ‘T’ section in the center of the switch is connected directly to the frog and bridges power from the right side to the left.  This connects the frog and the relevant exit rail or switched rail back to the black stock rail.

In the image below the turnout is set to the diverging route and the ‘T’ section connects the switched rail and frog back to the red stock rail.

To make the turnout non-power routing is a fairly simple fix.  I use two short sections of wire, as shown below.

These two wires are soldered to the copper plates as shown below.  The upper wire links the red stock rail to the diverging switched rail.  The lower wire links the black stock rail to the straight switched rail.

And that’s it.  This modification also makes the turnout even more reliable as the power is transferred through the new wires rather than the contacts in the ‘T’ sections.

With the base plate replaced the turnout is ready for use on a DCC layout.  It can still be used on a DC layout, the turnout simply won’t act as a power router. Also, if you’re not into soldering, this modification can be done away from your layout at a model club or possibly a local hobby store as the Kato turnouts will remain self-contained.

How to Fix Runaway Locomotives on a DCC Layout

When running your layout on DCC power have you ever had the problem of trains suddenly rocketing off down the track at full speed for no apparent reason?  Well a fellow modeler had just this problem this weekend.  So in this post I will explain what was causing his issue and what you can do to avoid it.

Before I can say why there’s a problem I need to explain a bit about how DCC works.  DCC powered trains all have a decoder inside which receives power and instructions through the track.  This combined supply is a 12V to 16V AC (Alternating Current) signal.  The decoder splits this into two separate parts.  The first part is the AC power which runs through a bridge rectifier.  This converts the AC power into 10V to 12V DC (Direct Current).  The DC is used to power the decoder and any outputs, such the motor and lights.  The second part takes the instructions, which are carried in the AC Bi-polar Square Wave as packets, and feeds them into the decoder processor.

(A Bi-polar Square Wave is not the same as a Sine Wave which you may have seen on an Oscilloscope screen trace; one is a series of square shaped variable width pulses and the other is smooth curved [Sinusoidal] and has a constant period time-base. The DCC signal as well as being square in shape has a variable time-base. By varying the width of each square wave pulse, a digital binary data bit can be transmitted. A binary 1 or a binary 0. It is the pattern of ones & zeros that define the DCC command being sent.).

The instructions will be things like increase speed or turn on light.  The DCC command station sends out many packets every second, that’s why the decoder can do many things at once.

A lot of decoders have the ability to run on traditional DC powered (Analog) layouts as well as DCC.  This is achieved by the processor understanding what type of power it’s receiving.  For example, if a locomotive with a suitable DCC decoder is put on a DC layout there will be no power applied until the DC throttle is turned on.  As the processor starts to receive a DC power supply but no information packets it realizes it’s on a DC controlled layout; this takes barely a second.  So it bypasses all of its complicated circuits and sends any DC power received directly to the motor and lights.  This makes the locomotive behave just like a normal DC locomotive.  It repeats this every time it’s moved on a DC layout.

The next time the locomotive is put on a DCC layout the second it receives an information packet it knows it’s on a DCC supply and returns to normal.

In an ideal world this works well and there should never be an issue, but things can go wrong and the primary cause of locomotives rocketing off down the track is short-circuits.  These are usually caused by derailing trains or when you’re putting rolling stock onto the layout whilst the track power is on.  Especially steam engines with lots of wheels!

So why does a short-circuit cause an issue?  When a DCC command station detects a short it turns the power off.  Some will keep trying to turn it back on or will require you to do it manually.  Situations where you have several quick short circuits, for example putting on a steam locomotive, can cause the command station to repeatedly start up and sending out its packet information as it turns the power back on.  If the decoder in the locomotive doesn’t receive a full packet it ignores it.  If this happens too many times on start-up it may get confused and think it’s receiving no packets of information and switch itself to DC.  The problem now is that it will bypass its processor and feed the full 10V to 12V DC from the bridge rectifier directly into the motor and the locomotive rockets off.

This situation can also happen if a train runs into a point or turnout which is set against it.  The system shorts, you change the point, the trains moves forward and shorts again as some wheels have derailed, you lift the derailed item, it shorts again but re-rails itself, the power comes on and other locomotives on the layout rocket off on a joy ride.

So what can you do to stop this? My advice would be to turn the DC running option off on all of your decoders.  This does mean they simply won’t work on a DC layout so bear that in mind if you run them on both.

So how do you do this?  If you have a computer connected to your layout or programming track it should be fairly easy.  Each brand of software is different but the principle is the same.  I use Decoder Pro from JMRI for my programming and the very first screen when you start programming a decoder looks like this.

Below the locomotive address options is the switch for turning off the DC operation.  In the advance setting or Comprehensive Programmer the option is in the basic tab and there is often a tab dedicated to just Analog Control.

But what if you don’t have a computer connected to your programming track?  The option to turn the DC on and off is contained within the CV (Configuration Variable) settings: CV no 29 controls this.  But it also controls the locomotive direction, the speed step settings, Railcom Settings, Speed Curve Settings, long address option and sometimes more, depending on the decoder.  So to work out what number to set CV29 to there are several calculators available on-line to work it out.  This page on Digitax’s website has several CV calculators and the second one down is for CV29.

If you are programing this CV change on an existing locomotive in your collection, rather than a brand new install, it’s a good idea to read CV29 first and see what the value is.  Then replicate this value in the calculator before making the change.  That way you won’t be changing something you don’t want to.

The 2mm Scale Association also has a good calculator here.

Some of the more expensive decoders are smart enough not to suffer from this but I tend to always turn DC off on them all, just to be safe.  Plus if you intend to install any Stay Alive systems to your locomotives you will need to turn it off anyway as a Stay Alive delivers DC power only and it could confuse the decoder again.

With all your locomotives set this way you should have a rocket free layout!