Cyclist is travelling at 12/60 = 0.2 miles/minute. It will take 6/0.2 = 30 minutes to reach the bridge. The cyclist running 25 minutes late, the train takes 5 minutes to reach the bridge, traveling 6 miles. Train is travelling at 6/5 = 1.2 miles/minute or 72 miles per hour.
But if it’s British Rail, there was a leaf on the line and the train was late by 4 hours.