ABS Problem

Every time I start my 92 GS Integra the ABS pumps starts running. After 1 mile down the road the light comes on and doesn't go off. Once I turn the car off and restart it, the entire process repeats. Is one of my sensors misaligned or gone bad? How do I check it? I'm sure its one of the front sensors. I happened after running over some railroad tracks. I need help in identifying the sensor itself and fixing it. Sorry for the long post. Thanks.