New Axles for a Second Generation Bachmann HO 4-8-4 Northern

Back in 2018 I shared with you my design for replacement axles and gears for a HO Bachmann 4-8-4 Northern.  You can find the post here. That turned out to be quite the adventure, as simply creating replacement parts didn’t work out so well!  Due to the thinness of the original components I had to engineer a much thicker gear which could handle the force applied from the motor.  This turned out to be a success.  But this gear and axle set only worked for the first generation of the Bachmann HO 4-8-4.   As it turns out, although Bachmann redesigned and improved the second generation, it too has problems with split axles and gears, and I’ve regularly been asked if I could produce a set for this locomotive.  Well, I finally have and in this post I’ll share the design with you.

To start, I want to explain the difference between the three generations that are commonly available.  I’m not going to go into any of the body or cosmetic differences, just the drive train so you can tell which generation you have.

The first generation, as I’ve already covered in other posts, has a pancake motor which drives the rear axle.  Below is an exploded schematic drawing, courtesy of John on the Tyco Depot Train Collectors & Model Railroading Forum. You can just about see the gear on the rear axle.

Looking at the underside of the locomotive there’s a bump over the rear axle to make way for the gear. 

This is the main clue that it’s a first generation.  This locomotive also has round wheel stubs and holes in the axles.  That’s what makes quartering the valve gear tricky on this locomotive.  ‘Quartering’ means setting the piston rods and side rods on each side of the locomotive at 90° to each other; as the wheel stubs are round it’s easy to get it in the wrong place meaning the loco won’t run smoothly.

The second generation had a lot of improvements such as a can motor and thicker gears and axles.  But most importantly the axles and wheel stubs also have square stubs and holes, making quartering far easier to do. You can just about make out the cracks in the axle and gear.

Below is an exploded schematic drawing, also courtesy of John.

The motor this time drives the third axle and there’s no bump in the base plate; this along with the lack of wires running to the tender tells you it’s a second generation.

The third generation, which I believe is the version currently available, had a total redesign.  It still has a can motor which drives the third axle, but everything else has changed.  Looking at the exploded schematic drawing below, again courtesy of John, you can see the axles are a different shape. 

To my knowledge, these don’t fail.  These are normally marked as DCC ready and will have wires connecting the tender to the locomotive; the DCC socket and speakers for sound are located in the tender.

Looking inside the second generation you can see the axles and drive gear on the third axle. The second axle is clearly split and once I removed the others I discovered they had split too. As the wheel stubs are square they don’t need to be such a tight fit as the round ones on the first generation.

Interestingly the axles are almost identical to the axles used on the UK Bachmann (former Mainline) designed locomotives that I have 3D printed replacements, for as shown below.  These were for a GWR Hall class and can be found here.

However, upon measuring the US 4-8-4 axles and gears I found them slightly different, so a new design was needed. These have been designed to be 3D printed in Shapeways Smooth Fine Detail Plastic.

The kit I’ll supply will contain three axles and a main drive gear. The idler gear between the worm and axle gear to my knowledge never breaks so I haven’t supplied that; if yours has, please let me know and I will add it to the kit.

These have now been sent off for a test print and I’ll see how well they work.  As with the first generation, and the UK Bachmann locos, I’m restricted by the size of the axle box and the size of the wheel stub.  This means the axle itself nearest the wheel is still not as thick as I would like it to be, and it’s possible to crack the new axles if not cleaned properly before use.  Once I’ve test fitted a few, if it appears too easy to crack the new ones I will include a few spares in each kit.

So now I’m waiting for the 3D printed parts to arrive, and I’ll share them with you when they do.

A Baldwin RT-624 in HO – Part 6

This week I have the final stages of testing to share for the HO Baldwin RT-624 before it’s released next week.

The big news is the etched brass has arrived. Thanks to all of you who pre ordered, your etch will be in the post this week.

The etch fret contains handrails, sun visors, grab irons, windscreen wipers, train phone antennas and MU hoses.

All the parts for the HO RT-624 kit look like this.

You may also need the truck conversions to rotate the Bowser trucks in the donor chassis as shown below. These are the same as for the HO DT6-6-2000 model and when I release the RT-624 next week I’ll include a link for these.

Lots of the details on the RT-624 etch are the same as the DT6-6-2000 but I still wanted to test fit as much as I could. Please note that in the photos the shell and 3D printed parts haven’t been fully cleaned yet and the brass is not glued in, just resting in place.

The end handrails are half etch and half 3D print. For strength the etch parts pass through the 3D print.

The sun visors fit nicely into the angled slots in the shell.

HO RT-624 - Brass Test Fit 06

The side handrails all have holes or locating lugs to fit into and it fits well, but as you can see its a little bent. This is simply because I got a little over enthusiastic cutting it out. As the etch is thick, 0.5mm, it takes some force to cut. I cut mine out on a cutting mat with a large craft knife (Box Cutter) and the force bent the handrails. The trick is to put something hard and flat on top of the etch to prevent the part you are cutting out from bending up.

HO RT-624 - Brass Test Fit 07
HO RT-624 - Brass Test Fit 08

The grab irons are basically the same as the DT6-6-2000 but I’ve made a slight change to the holes which made them a perfect fit. I fitted them without bending. Once pushed all the way in they easily bent down. Each grab iron has a small lug preventing them from being pushed in too far. When fitted with glue these will be secure and strong.

HO RT-624 - Brass Test Fit 09

All four on the side are the same.

HO RT-624 - Brass Test Fit 10

The top grab iron doesn’t bend.

HO RT-624 - Brass Test Fit 11

The last check I wanted to make from the etch was for the MU hoses. Each end has eight and they push into the square holes in the 3D printed section of the end handrail.

HO RT-624 - Brass Test Fit 12

It looks a little out of line as nothing is glued in place but once it’s all painted and securely fitted I think they’ll look very good.

HO RT-624 - Brass Test Fit 13

I now have a few small details I want to tidy up on the 3D printed shell but it’s almost finished. Next Monday I’ll release the model in two versions. The PPR early version with the Trainphone and the later without. There will also be one extra RT-624 for the single Minneapolis Northfield & Southern locomotive numbered Twenty Five.

A Baldwin RT-624 in HO – Part 5

My apologies for the lack of posts over the last month but this week I have an update to share with you on the Baldwin RT-624 project; the test print has arrived.

The photo above shows the shell and parts after they’ve spent twenty-four hours in white spirit which was used to remove the waxy residue left over from the print process. Some of this residue has turned to powder and can still be seen on the shell. To find out how I remove this see my ‘How To’ post here. I will be doing that next.

The 3D printed parts included with the shell are the crew in the cabs, horns, pilot blocks, Trainphone receiver, and end handrails centers with walkways. As you can see below the receiver and walkways have broken away. This is not an issue but I will update the 3D model to ensure this doesn’t happen again.

The receiver is designed to fit into a ‘n’ shaped slot at the top right of the nose; there’s only one on the loco, the other end doesn’t have one. You can just about see the slot in the loco shell below.

The receiver fits perfectly and once cleaned up can be permanently fixed in place with a touch of superglue.

On the pilot beam in front of the nose you can see four holes; these are for the end handrails centers.

These also fitted perfectly and have the holes to receive the brass handrail section which will complete the nose handrails. But both handrail center posts should have a walkway between them; this is the part that broke away and which I will ensure is 3D printed as one part in future prints.

The reason they came apart was an incorrect joint in the computer model which has been fixed. For this model the parts have simply been glued back together with superglue. Superglue is ideal for this because it’s made from acrylic, as is the 3D printed model. Other glues may have an effect on the 3D printed material and plastic glues may melt it.

The floor tread pattern actually faces towards the nose of the locomotive; this is because when it folds down to allow loco engineers to walk from one loco to another, the walkway pivots at the lower connection so the pattern will then be facing up. The face you will see looking at the nose will be the underside of the walkway. The square holes are for the MU (Multiple Unit) hoses which will be on the brass Additions fret.

With the handrail centre section repaired it again fits nicely into the holes in the pilot beam.

Looking from along side the locomotive you can see the walkway patten. You can also see the radiators in the side of the shell and I’m very pleased with the finish on those. You can also see I’ve added a little material under each handrail location along the side which will ensure each fits in the right spot.

Before I went any further I was keen to test fit the shell onto the chassis and did hit one design flaw. A different feature from the DT6-6-2000 is the RT-624 has a longer walkway on one side for extra batteries and a hollowed-out area. I copied the modelling for this from my N Scale model not thinking about the HO chassis and as you can see below the chassis hits it.

But that’s why I do test prints. The problem is easy to fix in the computer model and I can make a correction to this shell so that it’ll fit the chassis. I think apart from that, the shell is looking good.

I have ordered the first batch of etched brass Additions for this locomotive, and thank you to those who pre-ordered these. Once they arrive I’ll be able to do the final checks on the shell and, if all goes well, the HO Baldwin RT-624 will be made available to buy.

Hopefully next week I will have a cleaned up test print to share with you fitted to the chassis.

A Baldwin RT-624 In HO – Part 4

With the test print of the HO Baldwin RT-624 now in production at Shapeways, I’ve turned my attention to the etched brass parts that will accompany it. As with the HO DT6-6-2000 these will be in 0.5mm (0.0196″) brass and will come on a fret containing the parts for all versions of the locomotive.

The etch contains side handrails, end handrails, sun visors, grab irons, windscreen wipers, trainphone antennas and Multiple Unit hoses.

The sixteen MU hoses across the bottom of the fret fix into the 3D printed end rail section, as shown below, with eight at each end. All the brass parts have locating holes in the 3D printed shell for ease of assembly and strength.

As with the DT6-6-2000 the price for each fret will be £10 GBP and I’ll be looking for some pre-orders to help cover the cost of getting the etch tool made. If you’d like to be one of those pre-orders please drop me an email at or get in touch via the contacts page.

I hope the test print will be with me soon so I can share that with you and once the etched brass has been ordered the HO RT-624 will just about be ready to be released.

I’m still looking into 3D printed truck side frames for both the General Steel Castings Delta equalized trucks and the General Steel Castings Commonwealth trucks and hope to have those ready by the time the kit is available.

A Baldwin RT-624 In HO – Part 1

At the end of 2020 I released my HO model of the Baldwin DT6-6-2000, you can find the post here. But that wasn’t the end of the journey because just as Baldwin updated the DT6-6-2000 to the RT-624, so am I.

The Baldwin RT-624 is the natural development of the Baldwin DT6-6-2000.  Built between 1951 and 1954 the locomotive is classed as a transfer unit, designed for moving strings of freight cars between local yards in big cities.  It was a heavy locomotive, giving it great traction, and it had, for its time, a lot of horsepower.  Baldwin had started changing their naming convention for locomotives by this stage; this locomotive was sometimes called a DT6-6-2400 but Baldwin’s correct name was RT-624.  RT stands for ‘Road Transfer’, 624 stands for 6 wheels per truck and 2400 horsepower.  To achieve the extra 400 horsepower upgrade from the predecessor’s 2000 horsepower, Baldwin replaced the pair of 606SC supercharged diesel engines with their newer 606A supercharged diesel engines.  These each produced 1200 horsepower and were more reliable.

The RT-624 is very similar to the DT6-6-2000 and at first glance, you could be mistaken in thinking they were the same locomotive. Apart from things which you can’t see, such as the larger engine size in the RT-624, there are many differences. Below is my original model in N Scale of the DT6-6-2000.

Next we have the RT-624, again in N Scale.

The first thing that stands out to me is the large section under the cab that covers the fuel tank; on the RT-624 this has been removed. I assume Baldwin did this for ease of maintenance rather than cosmetic looks. Next, at the ends of the locomotive, Baldwin introduced a fold-down walkway to allow crews to pass between locomotives that were coupled together working as MU (multiple units). This split the end handrail as well as adding pipework for the MU equipment.

The trucks on some of the RT-624 locomotives were also upgraded to Outside Equalized trucks from the original Commonwealth trucks. Out of the twenty-four RT-624s made, twenty-three went to the Pennsylvania Railroad, and only the last eight, built-in 1952 and numbered 8724-8731, had the new tucks.

Another difference from the DT6-6-2000 was the walkways on the side. The DT6-6-2000 was almost symmetrical but the RT-624 had one walkway that was a different shape from the rest. This one had a longer raised section allowing for more battery storage under the walkway, which you can see on the left in the image below.

Another visual change that only appeared on some of the Pennsylvania Railroad locomotives was the Trainphone antenna on the roof. I say some of the locomotives because in doing my research I’ve found images of the locomotives with and without the antenna.  The Trainphone system was started in 1945 and was phased out around 1967 so it may be that it’d been removed from some of the locomotives. Below is a good example, PRR 8730 (one of the last batch with the Outside Equalized trucks) is shown with no Trainphone antenna. This photo was taken by Michael Sirotta at Frankford Jct., North Phila. It’s a 35 mm slide photo and dates from 1965 or ’66. (The image came from

The main body also had many small changes such as different grill positionings, headlight configuration, number board locations, and horn types.

The good news, as with my N Scale version, the donor chassis for both HO DT6-6-2000 and RT-624 kits will be the same. All the differences will be on the 3D Printed or etched brass parts.

I’ll be making shells available for versions with and without the Trainphone antenna for the PPR. I hadn’t planned on making 3D printed replacement trucks as Bowser makes both the Outside Equalized and Commonwealth trucks, but if they become hard to get hold of please let me know.

Out of the twenty four RT-624s made only one didn’t go to the RRP; this went to the Minneapolis Northfield & Southern and was numbered Twenty Five. Researching this locomotive, I noted that a lot of the changes above had not been included. As far as I can see, the only changes between MN&S’s original 5 DT6-6-2000’s and their only RT-624 was a different position to the headlight and a raised grill on the nose side. The large plate under the cab covering the fuel tank was already omitted on their DT6-6-2000’s. So for that model alone, if anybody would like one, I can simply modify the Baldwin DT6-6-2000 HO Type 3 model I already sell, so please get in touch.

I’ve already started working on the HO model and will have some progress images ready to share with you soon as well as a rendering of the model I’ll be sending for a test print.

Bachmann Mainline Replacement Wheel Centers – Part 3

A few weeks ago I received my first test print for my Bachmann Mainline replacement wheel centers, you can read the post here.  Although the first test prints looked good, one of the dimensions was incorrect, and consequently they didn’t fit.

This week I’ve received the second test print.  Again these have been printed in Shapeways Smooth Fine Detail Plastic.

Compared to the originals you can see there are four wheel centers with small counterweights and two with large ones.

This time the wheel centers fitted perfectly.  There was a little 3D print residue inside the crank peg hole which I cleaned out before test fitting.  The wheel centers are a snug fit and it’s worth bearing that in mind when painting; if the paint’s too thick it’ll prevent them from fitting correctly.

These can now be prepped for painting.  I do this by soaking the parts in Goo Gone for 24 hours, rinsing under a warm tap, and then leaving to dry for a few days.  Any 3D print residue left behind will turn to powder and can be brushed off.  I’ll then spray them black and re-assemble the locomotive.

As the wheel centers are a snug fit it’s unlikely they’ll fall out but as a precaution, I’ll put a few drops of superglue on the center of the wheel.  I wouldn’t recommend putting any around the outer rim or under the spokes as it’ll certainly spill out.

These are now ready to be ordered and can be found using the link below.

Bachmann Mainline Replacement Wheel Centers

These will fit the Bachman (Mainline) Split Chassis Standard 4MT, as well as any other locomotive that has a wheel diameter of 24.5mm (flange to flange) and an inside diameter of 20.6mm.

If you need a wheel center for a different locomotive, please let me know and I can make that available as well.