An Amtrak tunnel just south of the Baltimore station was abruptly shut down yesterday, forcing thousands of passengers on the main Boston-to-Washington rail line to use shuttle buses, after people living above the 108-year-old tunnel reported that their houses were cracking, shaking and shifting.

Amtrak officials said that they expect to complete pouring concrete to reinforce the tunnel early this morning and hope to resume passenger service by 7 a.m. along the heavily traveled northeast corridor route, which links Washington with Baltimore, Philadelphia, New York and Boston.

Baltimore fire officials were quoted by the wire services as saying five houses near the tunnel appeared to be in danger of collapse, forcing the evacuation of their occupants.

"One of them is definitely going to go," said Fire Capt. George Lloyd. "It looks like it's just a matter of time."

Lloyd said that yesterday morning "someone noticed a building with a crack in it, and things kept getting worse with time."

Authorities said the cause of the problem was not immediately clear. John McLeod, a spokesman for Amtrak, noted the tunnel's age, and pointed out that construction on the Baltimore subway is under way nearby.

After being alerted by police and fire departments, Amtrak sent an inspector into the tunnel, "who detected what could be an indication of ground movement," McLeod said.

According to another report, an Amtrak foreman who entered the tunnel "said he could hear the ground crack above him" and did not linger. "We immediately shut down all traffic," McLeod said.

Among those affected were passengers going to Baltimore for yesterday's running of the famed Preakness stakes.

After the tunnel was closed at around 10 a.m., southbound trains were stopped at the Baltimore station. Their passengers were taken by bus to the Baltimore-Washington Airport station and then brought to Washington by a commuter train. The process was reversed for northbound travelers.

"At least a couple of thousand" passengers were affected, McLeod said, and delays of an hour or more were reported.

After shutting down the tunnel, Amtrak sent in a team of engineers, who "found one place they thought might be affected" and decided it should be reinforced with concrete, McLeod said.

Although workers originally had expected to finish pouring the concrete by early evening, they encountered difficulty in reaching the spot believed affected, he said late last night.

After the concrete is poured, McLeod said a test train will be sent through the tunnel before passenger service resumes.

One Baltimore resident who lives near the tunnel said she is keenly aware of the passage of trains beneath her.

"When one of those trains goes through," said Elizabeth Knotts, "it almost shakes the pictures off the wall."

"When I have company and the dining room starts to rattle, they ask me, 'What was that?' I tell them, 'Don't worry, it's just the train,' " she said.