Plint:
I've had the same problem - going along fine then suddenly the pony truck (in front of the drive wheels) comes off the track. It turned out that the screw holding the pony truck on was put through a VERY-WELL-Machined hole - i.e. it was tight. I easily fixed it by using a metal drill bit and manually reamed out the ends of the two holes (top and bottom.) That usually fixed it, but if it was still a problem I also bored out the screw hole with a drill bit one size bigger than the hole.
As info, I've had a couple of steam locos illuminate the problem by the front of the engine just lifting the pony truck a little (from a little rise in the track) and when the pony truck came back down its wheels were derailed (especially on a curve.) Easy fix, see above.
Hope that helps.
K487
I've had the same problem - going along fine then suddenly the pony truck (in front of the drive wheels) comes off the track. It turned out that the screw holding the pony truck on was put through a VERY-WELL-Machined hole - i.e. it was tight. I easily fixed it by using a metal drill bit and manually reamed out the ends of the two holes (top and bottom.) That usually fixed it, but if it was still a problem I also bored out the screw hole with a drill bit one size bigger than the hole.
As info, I've had a couple of steam locos illuminate the problem by the front of the engine just lifting the pony truck a little (from a little rise in the track) and when the pony truck came back down its wheels were derailed (especially on a curve.) Easy fix, see above.
Hope that helps.
K487