The music is high tempo, the damsel is in distress, and the mustachioed villain is mugging to the camera. Yup, she's been chained to a railroad track, and the express is on its way. Used and, indeed, overused in many silent movies, to the point where it's been parodied for so long that no self respecting person would touch this Dead Horse Trope now. But when did that stop anyone? If a Dastardly Whiplash character appears, he'll probably use this trope without shame, befitting of a similarly "outdated" villain. For more information, see this page at The Straight Dope website.