An Oakland man is suspected of hitting a woman on top of the head with a blunt object while on a BART train Monday night, one of two attacks on women in about a 90-minute span on the transit system, police said.

Both women were expected to be OK, but one went to a hospital following the attacks on separate trains heading to the Colma and Orinda stations, respectively. The woman attacked in Colma suffered the more serious injuries, but they were not life-threatening, BART spokesman Jim Allison said.

Police arrested Duane Baines Jr., 29, on suspicion of attempted murder in the bus zone of the Colma station, after Baines got off the train, Allison said. Police did not specify the object Baines used.

The woman he is suspected of attacking went to San Francisco General Hospital and Trauma Center in an ambulance, Allison said. The attack on the Colma-bound train happened around 9:35 p.m. BART took the train out of service, so police could gather evidence.

In the second attack, around 11 p.m., 46-year-old Keith Montford, of San Francisco, is suspected of throwing a woman to the ground and robbing her of her backpack. Police arrested him at the Orinda station.

Baines remained in custody Monday at the the San Mateo County jail. Montford was booked into the Contra Costa County jail in Martinez but was not listed in custody Monday afternoon.

BART issued orders prohibiting both from using the system.