No News This Week

My apologies but there will be no post this week aside to say that today we lost a fellow modeler, so I know you’ll excuse me if I take this evening off.

Look after yourselves, and I’ll catch up with you all next week.

Following up on Messages

My website has now been up and running since November 2013 and in that time many people have contacted me with all sorts of questions which I try my best to answer, although sometimes I do get a bit behind.

However, every now and again I get a question come through via my contact page which I simply can’t answer.  This is not because I don’t know but because there is an issue with a mis-spelling of the supplied email address and I’m unable to send a reply.

So to help stop this I have added another line to my contact form asking you to re-enter your email, just as a check, which should help with the problem.

Alternatively you can always email me directly at  I do try to answer all emails, even if I can’t get to them straight away, but if you haven’t had a response and you think this might have happened them please get in contact again, I’m always pleased to hear from you.

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!

Small Steps Forward and New Releases on the Horizon

This week’s post will be short as it’s been a busy week.  I’ve made some progress with the UP rotary snow plow as you can see in the image below.  To read more about this project you can find the first post here.

As you can see a lot of the brass details have been modelled such as the grab irons and roof handrails.  Unlike most locomotives or freight cars, which have one point of access to the roof for maintenance, this rotary snow plow is covered in them.  So far there are 86 separate brass parts and there are many more to add.  But I know how much we, as modelers, love detail so if it’s right, it’s worth it.

On a different note; if anybody went to Amherst Railway Society Show, in West Springfield, Massachusetts, over the weekend you may have seen The Scale Architect’s stand with some of their new releases.  For those that couldn’t make it the new release which interested me the most was the ‘Naval Gun Barrel and F22 flatcars’.

This 3D printed kit will be available in March 2018.  Now I wonder who could have drawn that?!  A bit closer to the release date I’ll have some more info and pictures to share with you about it.

Next week I plan to get back to the UP rotary snow plow and I may even have the chassis worked out.

Union Pacific Rotary Snow Plow 900081 – Part 2

In this week’s post, as promised last week, I’m going to share with you which chassis I’m planning on using with my UP 900081 Rotary Snow Plow kit.

The reason why I plan on using a commercially available chassis is I want to make it easy to build and have reliable power pickup.  This will make powering the fan on the front much easier.  I can 3D print trucks, add contacts and fit metal wheels but when there’s something already available, which also has a motor and drive shafts, it seems the right way to go.  However the UP 900081, and UP 900082, have two different trucks front and back and the truck spacing doesn’t match anything else so it’s not going to be that easy.

The chassis I’ve chosen as a starting point is made by Kato and can be fitted in to all sorts of things but I think it was designed for Japanese street cars.  The 11-105 is the first of three including 11-106 and 11-107.  All three are the same just with different trucks.

The chassis are fairly cheap and available in lots of places online and in local hobby stores.  Plus they are great runners.

The plastic top is held on by four clips and simply pops off.  Underneath is a motor at one end and a worm gear and truck tower at the other.  There is a drive shaft connecting the two.

This means that only one truck in the chassis is powered by the motor, the other is free running but still picks up power, which is ideal for me.

With the top gear removed from the truck tower the chassis will become free rolling but maintain pickup from all the wheels.  This then allows the fan to be connected to the drive shaft.  This will also take a bit of figuring out as I want to incorporate a reduction gear box which will dramatically reduce the speed of the fan.  Although I intend to convert this to DCC, which will be very easy, simply telling the motor to run really slowly may cause it to stall or seem jerky. Having a gear box will allow the motor to run at a good speed whilst the fan slowly turns.

All that quality pickup will also make it easier to install sound as well, which is an interesting option.

As I said before, there are a few issues with the chassis. Firstly, the truck spacing.  This can be overcome by extending the chassis frame or simply 3D printing a longer one.

Secondly, the trucks themselves are the wrong type, and one doesn’t have enough wheels!  But this too can be overcome and again it will involve some 3D printed parts.  I haven’t decided yet whether to file down the existing side frame and stick on new ones or whether to 3D print a new truck outer section, but as the design develops that will become clear.  At this point you may be asking what is left of the original chassis?  Well, the motor and power pickup, which given the relative cheapness of the chassis is well worth it.

So I have a lot of design and drawing ahead of me on this project and next week hopefully I will have a bit more progress to share with you.

Union Pacific Rotary Snow Plow 900081 – Part 1

In last week’s post I told you about Rotary Snow Plows and my intention to make a model of UP 900081, the largest and most powerful rotary in the US.  You can find the post here.  In this week’s post I’ll go into a bit more detail about what I’m going to do.

UP 900081, as pictured below, is currently on display at the Museum of Transportation, St. Louis, Missouri, which is really useful as there are now lots of pictures available as reference material.

For my model the main body will be 3D printed in Shapeways FUD or FXD materials simply because to date this produces the best results.  And I’ll be setting the print orientation so the model is printed the right way up. (To see why this is important see my post about 3D print orientation here).

A lot of the surface area on the sides is flat so to help keep it smooth I’ll be making all the grab irons and ladders from etched brass. This has two advantages; firstly, anything sticking out of the side of the print needs support material underneath it in the print process and this can leave a shadow on the surface, and secondly, the etched parts look better. This has worked well on my other locomotives such as my Alco C855 as shown below.

This time I’m also going to include the mesh grills as an etched brass part.  The mesh frame will be 3D printed as part of the shell and a fine mesh will fit in behind.  This will allow you to see inside slightly as with the original.

Other details will also be made from etched brass such as the side window wipers and the unusual rotating windows on the front.  These special windows, shown in the photo below, rotated continuously preventing snow from settling and obstructing the windows.  (Photo of UP 900082 taken at Cheyenne 14th June 2000 by Don Strack),

In this close up on Don’s photo you can clearly see the rotating windows. My 3D printed shell will have a lip behind the window to receive a piece of clear plastic to form the glass and the etched rotating window will sit on top.

The big twelve-foot fan at the front will also be 3D printed but as a separate part as I want this to be able to rotate.  In fact in my model it will be powered. I will have more on how that will be done next week.

Above the fan is the directional chute which will also be a separate part so it can be flipped from left to right.

The trucks on the rotary are not standard either, well, the second one is as it’s recycled truck from a C&O steam tender, but I think the front truck was constructed from parts, making it unique to the rotary.

These will also be 3D printed but they will only be side frames as they will fit onto the chassis which will be a bought in.  Why not 3D print one?  Because I want good reliable power pickup and a motor to drive the fan.  The easiest way to do that is to use a powered chassis that works well.  I will also make a 3D printed chassis and trucks for anybody wants an unpowered version.

Behind the front truck is a snow plow which can be lowered to prevent snow building up under the fuel tank, but this is N Scale so that will be a 3D printed part of the shell.  It will also depend on how the new chassis fits in at that point.

In next week’s post I’ll share with you which chassis I’ll be using and how it will be connected up to the fan (if it arrives in the post in time).  For now I’ll leave you with a quick render of how my 3D model looks so far.