Yes. GP30,GP35,GP7, and GP9 come to mind.     Joe

Does the  back truck have 2 black wires on it? Normally, the red wire is from 1 rail and the black from the other rail. Bachmann might have used red for front and black for rear( or other way around). Trace the black wire from the rear truck to the  the board. Then from the front of the board to the motor tab.  Solder the red wire from the board to the other motor tab. Solder the red wire from the wheel pick up to the other tab. This way you should have a red and black wire from the same side of the frame (rail) to a motor tab.

Look at where wire 2 and 4 go to. Do both go to the top pad on the motor? If so wire 1 and 3 go to the other top pad on motor.

Is this a pancake motor style? One truck is powered only or 2 truck powered?
pancake motor:https://hoseeker.net/assemblyexplosionbachmann/bachmannemdgp40diagram1990.jpg
If so there are 2 pins on each side of the frame that go into holes in the shell. You need to spread the shell at these locations to get it off.
If 2 power trucks https://hoseeker.net/assemblyexplosionbachmann/bachmannemdgp40pg02.jpg
there is a tab on each side  of the fuel tank. Spread shell apart there to get it off.
how to post photos     https://www.bachmanntrains.com/home-usa/board/index.php/topic,33824.0.html

Buy this: https://www.makersource.store/products/digital-pull-meter        Joe

If you get a computer interface and JMRI go to youtube and search for dccguy. He has several videos on JMRI. He also has a website DCCGUY.COM     Joe

My friend has the MRC system you have. I use Digitrax. I bought him a Digitrax PR3 to use with a laptop as a stand alone programmer. These can be found  at on line stores.
  https://www.digitrax.com/products/computer-control/pr4/
MRC has one as well: https://www.modelrectifier.com/product-p/0001516.htm
You may have to do a search for this one.
And JMRI: https://www.jmri.org/   
Hope this helps.    Joe

The Bachmann DCC decoders are not the best in the world. Can you read CV's with your DCC system?If so read CV2 on the loco that starts on step 1. The long time to get up to speed is CV3. CV4 is deceleration or time it takes to stop when throttle is set to zero. Check CV3 and 4 on the other loco. Try and set the SD45's to these values.And what DCC system do you use? If it has a computer interface get one and down load JMRI. It's free and is a big help. You create a roster with all  your locos and save the info. If you have to do a re-sat on decoder, the info is all there. You just hit a button to program all CV's back to the decoder.   Joe

Make sure the keep alive is charged up. It can take up to a minute for it to happen. Also the keep alive only works for about 10 seconds or so then has to recharge.

To use JMRI, you need an interface between your computer and dcc system. If you have Digitrax, they have the LNWI :   https://www.digitrax.com/products/wireless/lnwi/
which doesn't require a computer or JMRI. The PR4 is used with JMRI and a computer. https://www.digitrax.com/products/computer-control/pr4/
Most DCC systems have computer inter faces as well.    Joe

Even on the prototype, curves were a problem!    Joe
https://www.youtube.com/watch?v=TO7xcQq0EXU

 NCE light decoder:  https://www.ncedcc.com/online-store/Light-It-3-3-pack-of-the-Light-It-Universal-Lighting-and-Signal-Decoder-p78626959
It's a 3 pack. With this one, you can hook up a pair of capacitors to prevent the lights from  flickering. NCE sells the caps. Only see them listed on their web site.  Joe
Caps:  https://www.ncedcc.com/online-store/search%3Fkeyword%3D5240306
I used 2 of these in my Cabooses.

I would say no on the 4-8-4. The GP40's  can be converted to DCC. Are you plan on DCC or DCC Sound? The 4-8-4 would be difficult.   On the GP40, NCE makes a non sound decoder : https://www.ncedcc.com/online-store/Bach-Dsl-replacement-decoder-for-Bachmann-p38322131   This should replace the board on the GP40. Sound decoders can be added as well, but the problem is finding room for the speaker.   Hope this helps.   Joe

Note: running DC locos on DCC will eventually burn out the motors.     Joe

 Scroll down the list of Tsunami decoders. There should be a Tsunami Steam Bachmann Sound Value list with all the locos listed. Remember to cycle the power off after re-setting the decoder. After a few seconds, the headlight should flash around 16 times to show re-set took place.   Joe