Thanks for all the info and answers. This set is in great shape for being around 20 years old! As near as I can tell mine has the version 4 chassis. Spent most of the day reading info from all the links given as well as many more.  Found out why the wheel wouldn't stay on. I took it apart to find the Chuff cam was laying sideways inside the wheel compartment so there was nothing for the axles to connect to. Ordered a new chassis yesterday for a complete rebuild.  In the meantime how are the wheels sychronised L to R? Should the rods be going forward at the same time, 90 degrees off, or 180 degrees off. Mine is currently at 180 and it wants to kind of catch every half revolution at slow speeds but smooths out somewhat at higher speeds. Haven't put it on track yet but just running it upside down with some jumper leads.

Just joined and this is my first post. Just picked up a used G Blue Comet set at a garage sale for $20 so couldn't pass it up. Been searching to find out the year range it was sold but coming up unsure. Anyway a couple of the drive wheels keep slipping out of the motor and need to find out what keeps the square end of the axle attached to the motor. Waiting until I get this figured out before I put power to it.
Wanting to set up a garden railway eventually so it seems I need to invest in some brass rail instead.
Thanks for any info or direction to my questions.