The thread and diameter can be found by taking the lead car to a well-stocked hardware store, and trying various screws until you find the correct one.
You probably something different than a hex head, but it would suffice as a last resort.
I believe those screws usually have a knurled head. Perhaps a machine shop could do that.
Do you know who made the track? Merriman? Schaefer? Harken?
Best regards,
Bud Brown
#1085
ECA Rules Vice Commodore