Athearn RTR SD40T-2 Project
Taking a stock Athearn Ready-to-Roll SD40-2, re-powering and adding DCC
  • Re-power with Kato HM-5 motor using my motor mount
  • Replacing head and tail light incandescent grain-of-wheat bulbs with LEDs using my filament
  • Adding ditch lights using SMD LED's
  • Add lighting to number boards
  • Add Tsunami2 DCC sound decoder
  • Install (2) of my sugar cube speakers, glued and wired together
Motor mount, 3mm LEDs, Headlight filament, and sugar cube speakers are available at 
The Athearn RTR ("Ready-to-Roll") line of diesel locomotives are an interesting evolution in their product history. They bridge the gap between BB ("Blue Box") locomotives and Genesis in repect to both timeline and detail level. They include many of the extra details that BB diesels lack such as grab irons and air hoses, as well as extra decals. The RTR diesels that I have examined contain the BB power hungry 1 amp motors, which are good candidates for re-powering. They are new enough to be "DCC Ready" in some cases. This particular exaple of an RTR SD40-2 is DCC ready, which I don't wind up taking advantage of. Read on!
Step 1: Disassembly and examination
Disassembly is a simple matter of removing the fuel tank and the couplers, and then slipping the shell off of the frame. You can see the head and tail light bulbs. This loco has 4 on the front and 4 on the back. They are secured with tape (!)
You can also see the "DCC ready" circuit board filled with resistors and diodes to give this DC locomotive directional lighting. There is both a NMRA 8 pin and a JST connector. This assembly is way too bulky to be useful, so it will be discarded. I am using a PNP Soundtraxx board which will replace this stock board.  
The stock board is riveted onto the motor assembly (see arrows). 

In this picture you can also see the typical Athearn and Proto solderless connections of all the wires (secured with small black plastic caps). These connections are unrelaible, so if you keep the stock board in place, it is highly recommended that you solder these connections.
Turns out that the board is riveted onto a metal strip that slips over the stock motor's top contact strip. This serves to deliver electricity from the board to the one of the motor's poles. The other pole's power comes via the black wire you see, which is soldered to the bottom motor contact.
With the board removed and the motor uncovered, you could almost mistake this loco for a BB model 20 years older. Also shown above the fram are the extra weights over the drive shafts, removed and sitting on either side of the DC board.
The motor removed. Although the motor is identical to a BB SD40-2, the frame is not. There is more metal cast in many areas, as well as those weights I removed.
Some of that extra metal is around the area of the motor. The KATO HM-5 motor (clipped into my mount) does not seat all the way into the frame. The frame clearly needs to be milled. But first, transfer the flywheels from the old motor to the KATO.
Step 2: Remove and install flywheels
This information can be found in a separate tutorial. The link below opens a new tab in your browser (where supported).
Step 3: Mill the frame
I believe some people just raise the motor up rather than grind the frame. I strongly disagree with that. I feel that the Kato driveshafts should locate at the same height as the stock motor to both save space and ensure proper operation.

Notice the front of the motor is to the right in this photo, which faces the rear of the loco. This means that the voltage of the motor needs to be reversed for proper connection (there is a CV for reversing the current of the motor, just in case you get it wrong).
A closer look at where the motor is hitting the frame. Actually, it is my mount that is hitting here. Why couldn't I design a mount that has a lower profile? Believe me, in the design I used the minimum of plastic required to hold the motor securely. 
Remove the truck assemblies and any other attached parts so you have a bare frame. You can see that I have started the grinding process with my rotary tool. It is trial-and-error here: grind, test fit, grind, test fit, etc. You need the motor handy to perform this task, so be careful not to get filings into the motor. I grind the frame in the garage and not at my indoor workstation for the same reason.
Grinding complete. Mount with motor now sits flat on the bottom.
Installation complete. It is important to test the motor and mount on a layout at this time. Here you see the motor wired to the pickup wires for testing. When I ran it on my test oval, high speeds resulted in a vibration sound. I sometimes encounter unbalanced flywheels on Athearn locos. Now you know why the motors are usually mounted on rubber mounts!
To try an mitigate some of the vibration transmitting to the frame, I added a neoprene spacer. It works a little, but the screws that hold the motor to the frame are still trasmitting the vibration to the frame. I suppose gluing the motor mount to the spacer, then the spacer to the frame would help more. For now, I will proceed as-is.
Step 4: Number boards
I like to separately light the number boards. In this loco, the number boards are opaque, so I will have to create new number boards that let the light shine through the numbers. First step is to remove the cab lid to access the area.
I created the new number boards using my Canon printer and the apropriate font. Here you see the different sizes of the font, I believe I wound up using 8-point.You can see the original number boards to the left.
After printing, I seal the ink and paper with clear gloss spray. Several coats.
As seen here I first started with my 1mm filament. As it turns out, there is enough space to push the tip of the bulb inside the assembly behind the number boards, so I increased the holes behind the number boards to 3mm. This will give a more uniform light source behind the number boards. During this process, I removed the headlight/number boards assembly from the cab.
You can see that the headlight assembly is removed here. I filed the rounded tip of the 3mm LEDs flat, which gave needed clearence and diffused the brightness a bit. I soldered some resistors on them, inserted them into the assembly, and soldered them together. This picture shows them removed after soldering. Note the common non-resitored lead is offset to the left. This is to make room for the headlights which go in-between the numberboard lights. When I soldered the lead connecting the two LED's, I was careful to give room for my headlights to pass through. 
The LEDs are inserted and I added some white paint for further light spreading. I also added black paint to the back of the LEDs to prevent unwanted light transmission into the cab. Then I glued the number board assembly back on to the cab. I sanded the existing print off the factory number boards, because underneath, they were clear plastic and perfect for me to glue my new numberboards to.
Here is the assembled number boards. You will see them illumiated at the end of this project.
Step 5: Head and tail lights, ditch lights
Note: I forgot to take pictures of the front headlight job, but read on because pretty much the same technique was used for the rear lights. For other examples and more detail about using filament with LEDs, check out my
This particular model has 4 front and 4 rear headlight lens holes. They were occupied by a whopping 8 grain-of-wheat incandescent bulbs. At the front of the loco, two of the headlight locations are between the number boards, and two more are located below the windscreen in the front of the hood. If you look closely in this picture you can see 2 pieces of my filament sticking out of the lower pair. I placed an LED behind them to shine through. I demonstrate this in the next step when I do the rear lights.
This picture also shows ditch light wiring. The ditch lights are very tiny SMD LED's with magnet wire soldered onto them. There are tutorials for soldering magnet wire to SMD LEDs elsewhere on the internet, but suffice it to say jewelry-level magnification is required.
Here is the rear lights location. I will put bulbs in all 4 and light them with a single LED using my filament. In many cases you can insert the new bulbs/filament from inside the shell, but in this case the bulb holes in the shell taper and the bulbs must be inserted from the outside.
If you loook at my plastic filament tutorial on this site, you will see an example of using the filament to light two headlights. This model has four, so I simply used 4 pieces. Here you see the 4 pieces of filament after the ends are heated next to a candle to create a bulb shape.
The LED and filament are attached with 4mm heat shrink. I did not shrink the tube to its minimum diameter. Trial-and-error, shrinking a little at a time, I got it to just the right diameter to fit all 4 securely. I don't want to bother with glue here.
Next I removed the filament from the assebly and inserted them in the shell from the outside. This view shows off the bulb ends of the filament in the shell.
View from the inside before attaching to the LED to the bare filament.
The technique for inserting the filament bulbs into the heat-shrink LED tube is straight-forward, but requires a little patience. You can see my long tweezers in there. Fitting 3 in the tube is easy, the 4th is the tricky part. Slide the whole filament/LED assembly rearward so the filament pokes out the rear of the shell, grab the long one in your fingers, twist and maneuver it until it works its way into the tube with the other 3. Then push all 4 bulbs into place from the outside.
After everything is in place, I just put a dab of hot-melt glue on the area of the LED to hold it in place. Once the dab of glue is there and still hot, I lick my findertip and squash the glue down a bit to make a good footprint on the shell's plastic. DO NOT put hot-melt glue in the area of the filment, because it will melt from the heat of the glue!
End result: a single LED lighting four tail light bulbs.
Step 6: Speakers and decoder
On other builds, I have put the decoder in the shell. I have since made a habit of putting the decoder on the chassis because it is easier to access. I am going to place the decoder in roughtly the same spot as the old DC board. The ventilation fans are right above this location, so attaching the speakers on top of the board seems the best idea. 
I am using two of my 13x18 High Quality Sugar Cube Speakers for this application. I want to glue two of them together and pass the wires from one to another through the inside of the enclosures. I just need two more holes in one enclosure to line up with the next. A hand drill with a tiny bit is just the thing.
I have soldered leads onto one speaker and inserted it into its baffle. No gluing yet. I make sure to get the +/- poles to match by identifying where the shorter of the two leads will attach on the second speaker (marked with an "S"). The polarity needs to be the same, or the speakers will cancel each other out and sound terrible.
I glue the first speaker in to its enclosure, pass the wires through, and glue the two baffles together. The loops I put in the leads are absolutely essential. The second speaker's single lead will solder onto the top pass-through lead. The second speaker's other contact will be soldered onto the larger loop at the bottom.
Not much space for the soldering iron? Sorry, I pressed it down a bit before remembering to take a picture. Testing this before gluing is absolutely essential, so connect it to your smart phone and turn on a game or music etc. Having a cut-off headphone jack off a pair of broken headphones is pretty handy right now. While the music is playing on the speaker, press it down into the enclosure to make sure nothing is shorting out and that it sounds right. When satisfied, glue the second speaker in.
Here we see the decoder installed with the speakers on top (loco is sitting on it's side). Before making all the shell lighting connections, I like to test the loco. On the left you can see 4 contacts, on the middle two I have soldered a resistor and LED for testing. I take it to my test oval and verify the motor's polarity by making sure the headlight is on when moving forward. The speakers sound great!
By the way, the speakers are attached to the board at two places. If you look closely, in the area marked with a circle, you will see the speaker leads soldered to the board's speaker output. This secures one corner of the enclosures. On the opposite corner I used a couple dabs of hot-melt glue to bridge the gap and make it secure. 
Here you see most of the shell's lighting wires attached. After making all the connections, I rest the shell on the chassis, and give it one more test on the track.
Fully assembled.
Pictured above is with the camera's flash on to see more detail. To the left is a picture with no flash to better see the headlight's glow. Looks like I need to touch up around the number boards with some black paint. I usually use a toothpick and lots of magnification. Oh, and it must be done while the loco is on the track with the number boards lit.
Thank you for visiting this tutorial!