EDIT: I noticed a small error while fixing the wording (lines "between" the cars where it should have been "on"), I fixed that and expanded on the explanation. Apolgoies for the confusion.

The pneumatic brakes on the train have two different "settings", one of them, used when parking a train, requires air pressure from the locomotive to keep brakes applied, while the other, used while driving, requires it to keep them released. The difference lies in the valves on the brakes switching between air from the pneumatic main line (locomotive pressure) and from pneumatic reservoirs on each car. I'll see to edit the article as soon as I can, clarifying that more than I did so far.