Well, as strange as the need sounds (awfully long screw for securing a truck on a Weaver car), I won't second guess your request. So here's what I do when I need something of this sort...
Find a hobby shop (like ours!) that caters to the R/C crowd. They sell threaded rod...2-56, 4-40, etc....in long lengths (typ. 12").
Cut a piece of the rod to the length you need.
Put a drop of thread-lock...or rubber cement...on the end you screw into the car.
Retain the truck with an appropriate nut/washer combo. Add a drop of thread-lock or rubber cement to help prevent the nut from working off over time with the swing of the truck.
But, back to the Weaver trucks... I don't have a pair of their RB die cast trucks on hand, but I do have several pairs of their Bettendorf trucks. A number 29 drill bit just clears the truck bolster hole. That's a bore of 0.136". A clearance hole for a 4-40 screw requires only a bore of 0.120". And that's the size screw I've always used on Weaver Bettendorf trucks, even if I have to increase the body bolster bore a tad...say, to a #43 drill bit (0.089"). As others have said, 2-56 seems really small for this situation.
4-40 screws are more readily available in longer lengths that 2-56 screws....especially in the cap style heads. But, again, if need be, you can get long lengths of threaded 4-40 rod for the length you need, and use a nut/washer to retain.