I made a voltage car (gunrunnerjohn's design and assistance) out of a Lionel boxcar. To get track power to measure the voltage of and also power the volt meters, I got some Lionel 4 wheel trucks with pick up rollers at my LHS (around $20 I think) and swapped out the original ones on the car.
If I could do it over, I would use super flex wire instead of the #18 or #20 stranded I used that tries to bind the trucks when making a sharp turn. The regular wire just isn't flexible enough. I think MTH and maybe Weaver may also have powered trucks, but my LHS didn't have them.