Well, that wasn't really my goal, but that's what happened. I put a DCC decoder in an old blue box Athearn GP38-2 loco - no problems there. I then had the bright idea to try and tune up the engine to improve it's performance, especially at lower speeds. Previously (i.e., ~15 y ago) I used this loco with a toy set power pack and I could only coax three speeds out of it: off, unrealistically fast, and even faster. So, I took apart the power trucks and gear assemblies, filed off a bit of flash from the gears, reassembled everything, and then turned on the power and ... (sorry, but nothing blew up, sparked, or made a big noise) ... found a problem that I either didn't have before or didn't notice. The truck assemblies (generally the front, but sometimes the back) shake from side to side (at several times per second) when the motor is turning at low speed. At higher speeds, this shaking disappears. Oh, when I am noticing this, the engine is held/balanced so the wheels are not on the tracks. I suspect that the rails and the weight of the engine would keep this from happening if the engine was placed on a track but that seems like covering up the problem rather than fixing it. To wrap up what is becoming a long-ish story, I disassembled/reassembled the power trucks many more times to check/clean/file the gears but the shaking did not go away. Eventually, I assembled the trucks without any of the gears except the worm that is attached (via couplings and shafts) to the motor/flywheels. The shaking still occurs. If I disconnect the motor and flywheels from the worm, there is no shaking so there's got to be a problem with the worm itself or the connection between the worm and motor/flywheels. This is getting frustrating! Does anyone have any ideas for specific things I should be looking for? Would lubrication help? Is trying to fix this shaking even something I should worry about? Thanks!