I have walked down to the big slide several times now and I think I figured out what caused it. When the railroad was built, they did it as cheaply as possible. This included several French drains. Over the years some have failed (plugged up). The slide looks to be in one of those areas. Logging a number of years ago folled in the drain and over time the ditch started filling up with silt. This retained water until it got to the point where the hillside was waterlogged. Then the weight of the trees took the hillside down.
Further down toward the right of way toward the brook, another french drain was affected the same way. Only this one took itself out of the right of way, leaving a washout.
As for the first washout, I don't know what was in there. I found some pins when we cut that area off. Possibly for a trestle. But there is absolutely nothing left if there was a trestle. There are no pipes from a culvert that used to be there. So this could have been another failed French drain.