Why are the gates at a railway crossing closed so early before the train actually arrives at the crossing?

- Jan 20, 2018-

The length of track over which a train is allowed must be reserved in advance of its movement on that section. So the sequence of events may go something like:

Level crossing gate is closed,

This information is relayed to the master of station in the rear,

Signal is lowered for a train waiting at (or approaching) the station,

The train begins to roll out from the station (or can move through it).

All of this might take a few minutes if the level crossing gate is a little distance away from the station.