IQ test

I like IQ test. Not to show i m so smart. But it has a lot of fun when you look for the answer.Recently,my friend who like this IQ test, ask me this:

There is City A and B. The distance within two city is 50km. There are two trains which will travel with 25km/h. There is a eagle which can fly 100km/h.

Train A will travel from city A->B while train B will travel from city B ->A.
When train A start travel, the eagle will start flying.
Now, there is a condition. When the eagle meet train B, it will fly toward train A again.
When the eagle meet train A, it will fly toward train B. The eagle repeat to do this until both train meet. the question is, how long have the eagle fly when both train meet?

Let Try!!