The Wild Engine

Helen, informed of the danger which menaces an excursion train because another engine on the same track is running wild, mounts a motorcycle and speeds down the track to warn the passengers of their imminent peril. Nearing a river trestle under repair, she hurtles into the river; undaunted, she swims to the opposite bank and flags down the excursion train in the nick of time, causing the engineer to run his train onto a siding just a few moments before the runaway dashes by. (imdb)