The Fly and The Trains Puzzle

Two trains are 200 km apart, heading towards each other, going at 50 km/hour each.

From one train a fly takes off, flying straight above the rails to the other train at 75 km/hour, bounces off it and flies back to the first train.

This is repeated till the trains crash together and the fly is smashed (Headline: Fly Dies in Freak Train Crash, Rest Survive)

What distance does the fly fly until its tragic end?

Note: there is a complicated and an easy way to solve this one.