If you have a postwar 027 uncoupling track with a single terminal underneath, the screw is either 3-48 or 4-40 machine screw, something like 1/4 or 3/8 inches long.
The original screws were binder head, but you should be OK with a pan head or even a round head screw.
Better hardware stores might have them.
I buy my screws in batches via mail order.