The Senate voted narrowly yesterday to uphold President Bush's veto of an Amtrak reauthorization bill that would have cleared the way for commuter rail service from Northern Virginia into the District. However, a parliamentary maneuver left the door open for another attempt to enact the legislation over Bush's objections.

The 64-to-36 roll call fell short of the needed two-thirds majority, preserving the president's perfect record in winning override votes on the 11 measures he has vetoed.

After the override attempt failed, Senate Majority Leader George J. Mitchell (D-Maine) switched his vote to the prevailing side, allowing him to ask the Senate to reconsider the vote at a later date. But in order to change the outcome, Mitchell would have to switch two votes -- a difficult task because no Democrat voted to sustain the veto.

Sen. J. James Exon (D-Neb.), who led the override effort, suggested that some votes may have been swayed because GOP lawmakers were heading to a black-tie 66th birthday dinner for Bush after the vote. "I hope that we can get back to not what is nice for a party but what is right for America," he said.

Last week, the House voted 294 to 123 to approve the Amtrak bill over the

president's objections. A two-thirds vote by both chambers is required to override a veto.

The lawmakers' failure to override the veto does not mean the end for the Virginia Railway Express. Virginia lawmakers said they hoped to attach the provision affecting the commuter rail service, which Bush does not oppose, to other legislation. Congressional action is needed to allow passengers to ride on Conrail tracks in the District to D.C.'s Union Station.

Yesterday's vote will not affect Amtrak operations, railroad officials said.

Although Bush said he supports the bulk of the bill, even those provisions face an uncertain future. Rep. John D. Dingell (D-Mich.), chairman of the House Energy and Commerce Committee, has said he doubts there is enough time to consider new legislation this year.

The president rejected the bill last month over a single provision that was aimed at clearing a vagary in the law by requiring

that the Interstate Commerce Commission review proposed acquisitions of the na- tion's 16 major freight railroads by com- panies that are not in the railroad busi- ness.

In his veto message, Bush called that mandate "an unprecedented new regulatory review requirement and . . . a step backward for the entire rail industry." Exon, the chairman of the Senate Commerce subcommittee on surface transportation, called the regulatory requirement "minuscule."

Currently, mergers of railroads and acquisitions of parts of railroads by other rail carriers are subject to ICC review. In addition, the commission has jurisdiction when a non-railroad company acquires either a segment of a rail line or two or more entire railroads.

The bill also included language that would have removed the final obstacle for Virginia Railway Express by exempting Conrail from liability in accidents involving the commuter trains on Conrail tracks. The commuter rail operation, scheduled to begin eight daily round trips October 1991, is expected to take 4,000 daily round-trip passengers off Northern Virginia's teeming highways, the equivalent of adding another lane to Interstate 95 at rush hour.

Without the release, Conrail will not

permit the trains from Fredericksburg and Manassas to cross the Potomac River into the District and to Union Station. In that case, the northern terminus would be Crystal City and the projected number of round-trip riders would drop to about 2,680 a

day.

Staff writer Don Phillips contributed to this report.