I have just purchased (a couple of weeks ago) a Bachman trolley car set. This comes with a length of track that causes back and forth movement of the train on it, with a pause at each end.. This is independent of the train -- I've used several small engines as a test.
I paid $100 for the set; Bachman lists it at $150 or the like. It needs special track which costs extra. The set comes with about 4 straights and 4 curves.
I don't understand the mechanism; it was all black box -- I just put it together and it worked.