[j7ake]

I think the problem here is posed in a way where there is a simple trick (because the fly flies exactly 2 times faster than one of the trains).

From the reference of a train, the other train is coming at 60 mph, which is equal to the speed of the fly.

So the problem would be equivalent to having the fly be stuck on the windshield of a 60 mph train until it collides with a train at 0 mph.

It takes 1 hour to move 60 miles at 60 mph. So the fly flies 60 miles.

If I recall in the original problem the fly flies at a different speed compared to the train (relative to the driver of the train), just to make it conceptually a little harder.