A northbound Conrail commuter train smashed into the rear of an Amtrak passenger train about one mile north of the Capital Beltway station last night, sending at least 39 persons to nearby hospitals.

Preliminary accounts indicated that none of the victims was critically injured.

The accident occurred about 6:25 p.m. at Seabrook Road in Lanham where the 15-car Amtrak Montrealer had slowed or stopped due to mechanical problems, according to Prince George's County police.

They said the four-car Conrail commuter train, northbound on the same track, was unable to stop in time.

About 200 people were on board the Montrealer, which left Union Station here at 6:10 p.m. bound for Montreal by way of New York City.The commuter train, which left the station five minutes later, carried about 250 people. The first scheduled stop was to be Bowie.

As of 8:30 p.m. yesterday, 33 persons had been taken to Doctor's Hospital of Prince George's County, and at least 6 others had been taken to Prince George's General Hospital.

At Doctor's Hospital, injuries were described as possible fractures and some facial cuts, which could require treatment by plastic surgeons.

Those taken to General Hospital were reported suffering from head and neck injuries. Full details were not immediately available.

According to police and witnesses, the Conrail train, each of whose four cars is electrically powered and self-propelled, smashed into a mail or baggage car at the rear of the Montrealer.

"I have no idea what caused the accident," an Amtrak spokeswoman said last night. She said it was under investigation by both Amtrak and Conrail.

The impact of the crash jarred several of the Montrealer's cars from the rails.

Ann Rudacille, of Manassas, a passenger on the Amtrak train, was in the dining car, third from the rear, when the accident occurred.

One moment, she said, she was eating, and "the next thing I knew, I was knocked across the dining room table.

Buses were being provided last night to take the uninjured passengers to Baltimore to resume their trip.