I think that the Great Central is addressing this in having visitor attractions spread along the line, with the NRM outpost to be built at the Southern end. This means that you will take a run on the train, have a break while you visit the museum and take a different train back. A lot of preserved lines have everything concentrated at one end, which sometimes deters visitors from actually travelling on the trains as you get on, go to the other end and there is nothing to do but stay on the train to come back. This is OK on a shorter run but not on a longer line.
Jim