The Legend: The protective ghosts of little children killed at a railway crossing push stalled cars off the tracks.
Basically, the story goes like this: In Texas, a school bus full of children stalls on the railroad tracks as a train was heading their way. Despite the efforts of the driver, the bus wouldn't start again and the train was coming too fast to get the children off. Everyone died. Now, if you stop on the railroad tracks, some say that the car will start rolling on its own. Small handprints that of children would be found on the back of your car. The spirits of the children guard the railroad crossing, making sure no one else suffers their fate.
Dating back to the 1970s, there are variations to the true origins of this legend. The story above is but only one version. Another one has a form of tame demon assisting the dead kids in their crusade. San Antonio claimed this legend to be its own but the actual bus accident took place in Salt Lake City. In the late 1930s, twenty-six children, ages ranging from 12 to 18, died when the school bus stalled on the tracks and was struck by a freight train, resulting in ten days of gruesome media coverage. Unfortunately, no similar accident has ever taken place in San Antonio.
The tale of the "ghosts of school kids that push vehicles off tracks" is actually a subset of Gravity Hill tales. Gravity Hill factlets really don't have a storyline to them (if a car is slipped into neutral at the right place, it'll move as if by magic). Gravity Hill even involves legends about dead teens which include cars stalling on train tracks and car rolls back uphill on freeway exit ramps (a carload of teens or a girl heading to the prom dies in a horrible accident on the off ramp). Of course, with that legend, there isn't any visual evidence. It's more like someone points out the weird "occurrence" and another volunteers the story.