Since trains move on fixed rails, they are uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which make it difficult to quickly stop when encountering an obstacle. Also, trains generally do not stop at level crossings, instead relying upon pedestrians and vehicles to clear the tracks when they pass. Therefore, from their beginnings locomotives have been equipped with loud horns or bells to warn vehicles or pedestrians that they are coming. Steam locomotives had steam whistles, operated from steam produced by their boilers.