If one train goes south 62kph but then another train goes north at the same time traveling 68kph then how lond will it take for the trains to be 455 miles apart

Visualize one train going north and the other going south.  To determine their distance apart, we could add the distance one train travels north to the distance that the other train travels south.

Thus, |62 kph * t| + |68 kph * t] = 455 km (Seems that you may be mixing km with miles; can't do that; choose one or the other.)

Then  

455 km
 = t = 3.5 hours
(62 kph + 68 kph)