EMD SD20
About the Prototype
The SD20 is the product of a rebuilding program by the Illinois Central Gulf’s Paducah Shops as a conversion from the EMD SD7, EMD SD24, SD24B (cabless), and EMD SD35 locomotives. The program involved rebuilding the 567 engine to 645E with 2000 horsepower rating, eliminating the turbocharger if equipped with one, upgrading the electronics to Dash 2 technology, and adding cabs to the B units. A total of 42 units were rebuilt. Original heritage of the rebuilt units was 3 Union Pacific SD7s, 4 Union Pacific SD24s, 10 Union Pacific SD24Bs, 21 Southern SD24s, and 4 Baltimore and Ohio SD35s. Many of the units to be rebuilt were purchased from Precision National. Three of the ex-Southern SD24s wore Precision reporting marks. The locomotives were rebuilt between August 1979 and December 1982. Road Numbers assigned were 2000–2041. Unit 2041 was the last Paducah rebuild.
(Source: Wikipedia: https://en.wikipedia.org/wiki/EMD_SD20)
The Model
I started this build by blocking out a basic model using the illustrations from Michael Eby (Trainiax) as a reference, in addition to taking measures from an Atlas SD24 high hood and a Proto2000 SD7 High Hood unit. Those two engines will be my donor engines.
So far the model is still missing detail. The side sill hasn’t been worked on, low hood detail is missing, the cab obviously isn’t there either yet, but I started adding the long hood detail, new fans and stacks.
Now there is a cab. I still havn’t worked on the side sill and the low hood. Further details may include an A/C unit on the cab roof, a K5 horn, removal of the low headlight, added ditch lights and possibly rock lights instead, a snow plow and more.
As this is a very custom unit, I will also add snow shields over the air intake behind the cab and possibly a winterisation hatch above the first radiator fan.
The model now has (optional) snow deflectors, ditch lights and preliminary rock lights (BC Rail style), as well as a snow plow and sun-shades. I added a sand-filler-hatch and high headlights. No handrails yet, as these will not be 3D-printed but wire-bent.
Offering 3D printed shells or the STL files for purchase
This model prints okay on an FDM printer at the moment, except for a very few optimizations I have yet to implement. The Long Hood wall thinckness could use another .4 mm for stability.
The long radiator frame at the rear has three posts, but the smaller 1st and 3rd post won’t print. Maybe, that part would be a great separate detail along with the grille’s mesh structure.
The cab is now a separate part.
I found this type of model on etsy for a lot of money, as apparently it is the only one available for model rail roaders at this time. So I’m thinking about offering either the file or a printed model via shapeways or another fulfilment service if there is some interest.
If there will be a model for purchase, this will very likely be the shell only. As I am basing my model on two separate chassis (a Proto2000 SD9 and an old Atlas SD24), it remains to be seen, how this can be made fit. It will probably be two separate kits. The Proto2000 Chassis is probably the better choice, as it features a separately applied fuel tank, which could be replaced with a more prototypical tank. It’s also a good runner. The old Atlas frame needs to be milled, to allow for a separate tank. Alternatively it could be milled to represent the prototype shape a little better. But either way, it needs to be milled.
So the Proto2000 based kit might include the Shell and the fuel tank next to the detail parts. The Atlas SD24 chassis on the other hand with its cast-on fuel tank, would either require milling the chassis or some sort of glue-on side-covers to more or less re-create the general look of the SD20 tank. Also the shells will not be interchangable.
Anyway, I would say the SD9 is the better model base, for its running qualities and greater flexibiity. The SD24 is currently my donor chassis, so I’m kinda developing for that one at the moment. I will likely finish the development on that first and then turn to the Proto2000 version in a few weeks. More news, if I have them.
Update May 2025
I finally got to work on the chassis again. So far it lacked coupler pockets and mounting posts. Also I added holes on the inside of the side sill, where the Atlas frame’s mounting retentions snap into, to secure the shell to the frame. A small boolean error also resulted in the pilot stairwell’s truck facing wall being straight and not slanted like the stair facing wall, resulting in less clearance for the trucks.
With these items fixed, I have now two frames on the printbed to see whether I erred on the coupler height, which I usually do on first iterations.
Next, I added the cable channels for the LED lighting of the Loco. So far I added these for the Rock Lights, Ditchlight Risers, the front pilot platform light (should be on the rear too). The front and rear step lights and the headlights. The Truck Lights actually don’t need channels, as the space under the cab is plentiful.
Coming up are some pictures of the preliminary shell. The paint is only a cover test to see how many layers might be needed. Obviously it’s only the first layer. I experimented with a 20-mesh screen-door cut-out for the radiator grille, but it’s a bit too wide. I have 40-mesh and 60-mesh on order, which will likely look much better. It also needs to be cut in a diagonal pattern.
The mesh insets have been optimised to get the mesh closer to the separately applied radiator frame. The next iteration will show these improvements.
So far the shell has been set onto the chassis and sees some testing on the test track.
