As the Senate Finance Committee moved yesterday toward passing what would be the most radical tax overhaul in a generation, Sen. David H. Pryor (D-Ark.) likened the fast-moving bill to a train.

"For months," he observed, "we've had passengers get off this train and we've had passengers get on. Now we've got a locomotive -- this promise of a 27 percent top tax rate."

And yesterday, the train left the station.

Committee Chairman Bob Packwood (R-Ore.) and an unusual coalition beat back one amendment after another, causing once-hesitant panel members to clamber aboard in support of a dramatic plan to slash tax rates, wipe out many widely used deductions and increase corporate taxes about $100 billion by 1991. The committee approved the legislation unanimously.

"This bill went from immovable to unstoppable in 24 hours," said a Packwood aide.

The train was fueled by a blend of politics, principle and pride, according to members and aides. Only last month, the panel was voting for one tax break after another for businesses and individuals -- an approach that seemed likely to doom the movement for lower tax rates.

Packwood got his members' attention by proposing a near-halving of the top individual tax rate, from 50 percent to 27 percent. Sen. Daniel Patrick Moynihan (D-N.Y.) said members also were swayed by a report showing that W.R. Grace and Co., a profitable multinational firm, paid taxes to Libya for oil-drilling operations, but owed no net taxes in the United States.

"The reputation of the American business community is not going to survive that kind of thing," Moynihan said. "There's a feeling that we're finally doing the right thing here."

Some Republican members, including Sen. Charles Grassley (R-Iowa), said they wanted to help Packwood match the feat of Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.), who moved a tax-overhaul package through his panel and the House. Others said they simply didn't want the president's No. 1 domestic policy initiative to die on their doorstep.

Still others said they were sick of reading newspaper articles saying that the Finance Committee could not produce a tax-overhaul bill. "A number of us take pride in the fact that the lobbyists are all stunned, they're shellshocked," said Sen. Max Baucus (D-Mont.). "We really want a bill. . . . I think it's going to have a sizable majority."

Only two days ago, the prevailing question among lobbyists and senators was whether Packwood could recruit the 11 Finance Committee votes needed to move his package to the Senate floor. (He had seven public commitments by week's end.) But yesterday, the question was outdated.

Asked if he was number 11, Sen. Charles Grassley (R-Iowa), responded: "I might be the eleventh, but if I am, there will be a 14, 15, 16 and 17."

"There's a dynamic," said Moynihan. "Once you get 11, you get 15. People want to be with you."

Packwood beat amendments with a core group of firm supporters -- mainly Sens. Bill Bradley (D-N.J.), John H. Chafee (R-R.I.), John C. Danforth (R-Mo.) and George J. Mitchell (D-Maine) -- supplemented by a shifting group of other senators, depending on which industries and groups were affected by a particular amendment.

The second group included Baucus, Moynihan, Senate Majority Leader Robert J. Dole (R-Kan.) and Sen. David F. Durenberger (R-Minn.), among others.

The issue that most threatened Packwood's coalition was his proposal to crack down on tax shelters. But Dole, Baucus, Sen. Lloyd Bentsen (D-Tex.) and others from oil-producing states said Packwood's plan would hurt legitimate oil drilling partnerships and would have to be changed in order to win their vote.

Late last night, the panel compromised, protecting certain oil partnerships. That won over the panel's powerful oil state bloc, but prompted Bradley to predict that the measure would "draw tax shelter dollars to the oil and gas industry."

"We had a principle of ending shelters," Moynihan said. "Like most principles around here, it ends up getting compromised."

"We've taken care of insurance [firms] . . . defense contractors," Dole said. "We're asking for some help for very small producers who [in Kansas] produce about three barrels a day."

Although tax overhaul finally pulled out of the Finance Committee, its future is uncertain. Senators supporting Packwood in the committee are not bound to support him on the floor, where the rules allow senators to offer unlimited amendments.

Sen. William V. Roth Jr. (R-Del.) yesterday withdrew an amendment to restore Individual Retirement Account deductions but vowed to bring it up on the floor. Real estate lobbyists expressed hope of paring the anti-shelter provision on the Senate floor, too. Other groups had similar plans.

To all of them, tax-overhaul advocate and former basketball star Bradley had one response: "One game at a time."