The best way to solve this is to make an algebraic expression to represent the situation~
If x is the speed of the first train and y is the speed of the second we can make 716.7 = x + y
the first train is 5.3 mph faster so x = y + 5.3
Now we can replace x with our new value to get 716.7 = y + (y+5.3)
Simplify and we get 716.7 = 2y + 5.3
Now all we got to do is simplify this equation, 716.7 - 5.3 = 711.4. Divide by 2 to get 355.7 for the speed of the second train. Since the first train is 5.3 mph faster, we add that to the second trains value and get our answer. The first train is going at 361.7, the second at 355.7