Amtrak may carry as many as 27 million passengers, like these at Washington’s Union Station.

NYT / DANIEL ROSENBAUM

WASHINGTON — Record prices for gasoline and jet fuel should be good news for Amtrak, as travelers look for alternatives to cut the cost of driving and flying.

And they are good news, up to a point.

Amtrak set records last month, both for the number of passengers it carried and for ticket revenues — all the more remarkable because May is not usually a strong travel month.

But the railroad, and its suppliers, have shrunk so much, largely because of financial constraints, that they would have difficulty growing quickly to meet the demand.

Many long-distance trains are already sold out for some days this summer. Want to take Amtrak’s daily Crescent train from New York to New Orleans? It is sold out on July 5, 6, 7 and 8. Seattle to Vancouver, British Columbia, on July 5? The train is sold out, but Amtrak will sell you a bus ticket.

“We’re starting to bump up against our own capacity constraints,” said R. Clifford Black, a spokesman for Amtrak.

The problem is that rail has shriveled. The number of “passenger miles” traveled on intercity rail has dropped by about two-thirds since 1960, and the companies that build rail cars and locomotives have also shrunk, making it hard to expand.

In 1970, the year that Congress created Amtrak by consolidating the passenger operations of freight railroads, the airlines were about 17 times larger than the railroads, measured by passenger miles traveled; now they are more than 100 times larger. Highway travel was then about 330 times larger; now it is more than 900 times larger.

Today Amtrak has 632 usable rail cars, and dozens more that are worn out or damaged but could be reconditioned and put into service at a cost of several hundred thousand dollars each.

And Amtrak needs to buy new rail cars soon. Its Amfleet cars, the ones recognizable to riders as the old Metroliners, are more than 30 years old. And the Acela trains have about a million miles on them.

Writing specifications for bids, picking vendors and waiting for delivery takes years, even if the money is in hand.

Amtrak is an alternative to airlines along the Boston-New York-Washington corridor, and on some routes out of Chicago and a few in California. But most of its other routes are so slow that people take those trains because they have no alternative to reach places like Burlington, N.C., or Burlington, Iowa. Or they go for the train ride itself.

The railroad carried about 25 million passengers last year and may hit 27 million this year. (That is all intercity traffic; commuter rail, connecting suburbs and cities, is also growing, but that is not Amtrak’s market.) By contrast, the airlines carry about 680 million domestic passengers a year. If Amtrak were an airline, in terms of passenger boardings it would rank approximately eighth, behind Continental and US Airways and ahead of AirTran and JetBlue.

H. Glenn Scammel, a former head of staff of the rail subcommittee of the House Transportation and Infrastructure Committee, said the railroad should give up on some of its cross-country trains and redeploy the equipment on relatively short intercity trips.

But the railroad’s labor contracts provide stiff penalties for dropping routes, and dropping states from its itinerary would hurt its political support, especially in the Senate, where thinly populated states are overrepresented relative to their population.

Scarcity is not all bad for the railroad, though. It has raised ticket prices, so that it recorded ticket revenues of $153.4 million last month, up 15.6 percent from $132.7 million in May 2006. That jump is higher than the ridership increase of 12.3 percent, to 2.58 million, from 2.30 million.

Despite its popularity with passengers, the biggest determinant of the railroad’s health is still the federal government, and in Washington, views diverge sharply.

Last year Sen. Frank Lautenberg, D-N.J., and others won overwhelming Senate approval for a bill that would offer the states 80 cents for every 20 cents they spend on new intercity passenger rail service, the same as the match offered for highway projects.

The House passed a bill with the same provision by a veto-proof margin earlier this month. That bill will soon go to a conference committee, but the White House is threatening a veto because it wants the passenger rail system to be turned over to private operators.