If you and I stand back to back at a light post and I run away at 10 km/hr and you run away at 15 km/hr relative to the light post, then we are running away (10+15) km/hr relative to each other. That means the distance between us is increasing at a rate of 25 km/hr.
So if the two trains are leaving the station at those speeds relative to each other, how fast is the distance between them increasing? Using this combined speed, how long will it take to get to 480 km apart?
Stephen La Rocque.>