My experience is from a shipper point of view, but my thoughts were something between 3 and 6 trains per week to make track viable as well.

In terms of rail cars even at 30 to 40 trains per year you would only be able to manage with 100 cars if you had a very efficient unit train service (all cars stay together from origin to destination and back). My 100 car number was given just to give an order of magnitude of what a train move would handle. In terms of movements in manifest service 8 turns per year is about normal for rail cars. Probably looking at more than 200 cars, to allow for loading unloading and full and empty transit times, even in a best case senario. It wouldn't change the overall economics much though as the track would be the big capital item.