Hi Tiffany, I have the RK Challenger and I pull a twenty car frieght consists up my thirty foot long five percent grade with her. There are two 180 degree turns on that grade (it is really an elongated helix) and I never had any problems with her getting the job done.
In fact I bet it could do alot more but with such a grade I doubt if that would be a very good idea for the longevity of the engine.
In a flat, or 2% graded world fifty to seventy-five cars should be fine.