I have had success with the truck.
The top of the truck mount is plastic and the pieces can be pressed together and inserted in the frame mount. Success! 

I have received today a second hand HO Bachmann Doodlebug.
The front of the unit was sitting lopsided, and on investigation, I found truck had come out of one side of the rounded mounting slot, with the drive shaft also loose from the motor.
I am reluctant to push the rounded mounting of the truck too hard in case I break the frame.
How can the truck be fitted back into the frame safely?
Regards
Trevor