Torpedo is a small charge which can be strapped onto a rail to alert the engineer of trouble ahead. The largest is about the size of a man’s wallet the smallest not much larger than a fifty-cent coin
In the days before two-way radio communion, there was no way to alert the engineer of a train that trouble lay on the tracks ahead.
Flares might be used at night but were useless in the day. In 1841 Edward Alfred Cowper invented the small explosive device. Strapped to the rail by the attached lead straps it would explode. with a loud “bang” when locomotive rolled over it. This would alert the engineer who stopped the train.
Torpedoes are still in use in some foreign countries and are still manufactured in this country, but the advent of radio communication and the fact that modern diesel cabs are completely enclosed and virtually soundproof makes the use of torpedoes a thing of the past.