A 10-wheel dump truck veered through a guard rail on a bridge in Germantown early yesterday and toppled 30 feet onto railroad tracks, injuring the driver and delaying commuters for hours, police said.

The accident happened about 6:40 a.m. as the truck, driven by Irene Czypera, 22, of Rockville, was making a right turn from Waters Road onto the Rte. 118 bridge over CSX tracks just west of the Germantown rail depot in upper Montgomery County, police said.

Czypera was listed in serious condition at Suburban Hospital in Bethesda with a broken right leg and pelvis.

About 1,400 passengers on four commuter trains that originated at Brunswick, Md., and Martinsburg, W.Va., were delayed as much as 2 1/2 hours, waiting for other trains or riding Montgomery County buses from the accident site to the Shady Grove Metro stop, county and rail officials said. An Amtrak passenger train from Chicago to Washington was delayed 50 minutes.

Traffic on Rte. 118 was halted until about 2:30 p.m., when bridge guard rails were replaced.

Police and officials from Vemart Contracting Inc., the excavation company that employs Czypera, said they did not yet know why the empty truck went off the bridge.

Czypera was not wearing a seat belt at the time of the accident, according to police.