To solve this question, we will have to use the concept of relative speed. 

Let us assume that the distance between the station A and B is D Miles
The relative speed at which the train are approaching each other is 20 + 15 = 35 mph
When the train collides with each other, the sum of the distance covered will be the distance between the stations i.e. D.
Now Speed = Distance/Time
Which means that the trains will collide in d/35 hours after their start

Another constant things we have here is the speed of the vulture i.e. 25 mph.
Thus by the time the trains collide, the vulture would have covered
25*(D/35) miles = 5D/7 Miles