Sen. Charles McC. Mathias Jr. (R-Md.) took a train ride yesterday from Union Station to Baltimore to display his opposition to the Reagan Administration's recent move to halt federal subsidies for Amtrak.

"If Amtrak funding is eliminated from the budget, Amtrak will go bankrupt and service will terminate," Mathias warned as he boarded Amtrak's Virginian for the 41-minute trip. He said the plan would lead to higher costs for state governments and "substantial disruption" for commuters and other travelers.

Mathias was joined on the 9:30 a.m. ride by Sen. Claiborne Pell (D-R.I.), who predicted "chaos" without Amtrak. They were given a sendoff by three other Amtrak advocates -- Sens. Joseph R. Biden Jr. (D-Del.), Bill Bradley (D-N.J.) and Frank R. Lautenberg (D-N.J.) -- at a news conference at Union Station.

The administration's move to end Amtrak's annual $684 million in subsidies has sparked widespread protest from passengers and politicians. Without federal aid, Amtrak officials have said, the railroad will go out of business. The administration has challenged this assertion.

The proposal, included in the administration's deficit-curbing recommendations for fiscal 1986, already has encountered sharp opposition in Congress. The issue is expected to be debated next week when the Senate takes up a budget compromise drafted by Senate Republicans and the White House.

Eliminating Amtrak would have a severe impact in the Washington area, the railroad's supporters say. Sixty-eight Amtrak trains stop daily at Union Station, which is slated for multimillion-dollar renovation. The station is used by nearly 2.3 million Amtrak passengers annually.

Maryland officials have warned that commuter rail service to Washington from Baltimore and Western Maryland would be halted if Amtrak went out of business. One of the commuter system's three lines uses Amtrak tracks and all three lines depend on Amtrak for maintenance and other services.

The commuter system, known as MARC (Maryland Rail Commuter), is used by about 6,300 passengers a day, according to a Maryland Railroad Administration spokesman. The system also has been jeopardized by an administration plan to end mass transit subsidies, including $2 million a year for MARC, the spokesman said.

The senators yesterday praised Amtrak's "impressive" service. They warned that highways and airports would be more congested if Amtrak shut down.

During the train ride, Mathias and Pell handed out questionnaires to about 200 passengers, asking how they viewed Amtrak's prospective demise. "They all said, 'Don't cut off this train. Don't stop the funding for Amtrak,' " a Mathias spokesman said later.