Answer:
60 miles per hour
Explanation:

You divide 300 by 5 and you get 60. You always need to divide the upper part to the lower part to reach the distance in ONE UNIT OF TIME. There, you have 5 seconds, yet you need the distance in 1 second.
This all is assuming that the train moved at a constant speed, where
ACCELERATION = 0, which is pretty improbable for a train.