Thank you all for your replies. It guided me in the right direction. When I removed the old trucks I noticed U-shaped leaf springs on each side of the trucks. When I put the new trucks in, I didn't pay close attention to their position. It became obvious to me upon further examination that these leaf springs are what transfers the power from the wheels to the motor. That led me to carefully position them so solid contact was being made and what do you know, the train worked just fine.

thanks again!

Thank you for your reply. I did place the four washers back in place when I put the train back together. I also tried to run it without its case on it. I do not know if that'll work in a normal situation, but it also did not work. Thanks.

I was using my Bachmann Spectrum locomotive (Fairbanks Morse H16-44Diesal) and all at once the motor was spinning but the train did not move. I discovered that the drive shaft that transfers motion from the motor to the rear gear train had broken. Bachmann doesn't sell the new drive shaft for that model locomotive so I ordered two new replacement trucks for it. I got them today and very excitedly removed the two old trucks and put in the two new trucks and quickly took it to my layout to test it out. Sadly, it did not run at all. In fact, it caused my power supply to pop its breaker several times. I removed the new trucks and reversed them, front to back and vice-versa, but still it doesn't run. Any thoughts on why it wouldn't work. It seems very straightforward to me. But I am a novice on this. I hope I did not ruin my locomotive.