The goal is to answer the question of "is the entire train that passed into this block now out of the block?", alternatively phrased as, "has the train lost any

The goal is to answer the question of "is the entire train that passed into this block now out of the block?", alternatively phrased as, "has the train lost any wagons in this block?". If anything has got left behind, you can't let anything go into the block.