You could look for piano wire that would fit inside the gear and cut to fit. Or buy a new truck.  Joe

The new Ecnomi board style is the way I would go. Replace the PC board in there now with it. Also look to add a current keeper to it as well. Part number 810140. Joe

http://www.soundtraxx.com/dsd/econami/ecopnp.php

Although for a GP9 this will give you an idea on how to install a decoder.
http://www.soundtraxx.com/documents/appnotes/bachmann_gp7.pdf

What brand of loco? Is it DCC friendly? IE motor isolated from frame?  Joe

I have a walk around companion, 2 plug in ports and the cables. If interested, reply and I will contact you by PM .  Joe

What system? Some have settings for N,HO, and O scales which will put out different voltages.  Joe

Make sure the 0 is on the Zypher display when you push the F0 button. Is F7 on? This is a dimming function for the lights. Look at the headlight with the direction set to forward. Any light? Then try reverse. Any light? If the headlight comes on in reverse, you need to change the value in CV29. Read CV29, if it's an even number subtract 1,if odd add 1 to the value. The other thing would be to remove the tender shell and find the board with the lights wires to it and check for voltage with the loco on the tracks and F0 on.   http://estore.bachmanntrains.com/index.php?main_page=popup_image&pID=1100
  Joe

 Make sure the wire harnesses from the loco to the tender are both in tight. If they are you may need to remove the tender shell and check the wires to the decoder.  Joe

http://estore.bachmanntrains.com/index.php?main_page=popup_image&pID=1283
how about the above or buy a frame w motor
http://estore.bachmanntrains.com/index.php?main_page=popup_image&pID=5930
Joe

http://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=66_68_183&products_id=1267

Will this work?   Joe

 Contact Soundtraxx for help. I believe this decoder has a cap on it already and might bot be suited for an add on.  Joe

 Bachmann makes 3 styles of GP40 locos: DC ,DCC ready and DCC soundvalue. On DCC ready, there is an 8 pin plug on the PC board for a decoder. Take the shell off and see what it looks like inside. It should have a PC board with 6 wires: 2 for the motor,2 for the front truck, and 2 for the rear truck. If it does, NCE makes a good decoder for Bachmann loco's. The train set loco proably has light bulbs. The NCE decoder is for LED's. If you go thet route, buy some 3mm leds.  Joe
go here:  https://ncedcc.zendesk.com/hc/en-us/articles/200945019-BACH-DSL
down load the install manual. 

Put both units on the track about a foot or so apart. Run both units to see if the B unit is running faster then the A unit. If so the B unit needs to be slowed down or the A unit sped up. This is where speed matching comes in. I would replace the decoders if they're the Bachmann ones with the NCE ones.   Joe

 Look at this:    http://www.modelrectifier.com/resources/dc-ac/Tech%206%202.0%20%200001200.pdf
  Joe

 How many locos do you have or plan on getting ? A Digitrax PR3 Xtra might be worth a look. You hook it up to a computer and use it to program decoders using JMRI a free software. JMRI lets you create a roster with all your locos so if you have to reset the decoder, you have a file of the CV values.
I would like to see photos of your NP RS3 .  Joe

http://tonystrains.com/cv29-look-up-table/
  Here is a link to CV29 values. On Digitrax  128 and up is considered a 4 digit address.   Joe