The Fly and Two Trains

Two trains are on the same track, 100 km apart, heading toward each other. Each travels at 50 km/h.

A fly starts at the nose of one train, flies at 75 km/h toward the other, instantly turns around upon touching it, flies back to the first train, and continues bouncing between them until the trains crash and squash it.

How far does the fly travel in total?