If you've always wanted to ride a train across the country but didn't have the time to take it all the way back, United Airlines and Amtrak believe they have just the deal you've been waiting for.

They unveiled a train-plane ticketing agreement this week that will offer passengers a cheaper fare when they fly one way and ride the rails the other way.The setup would also allow travelers to fly into certain major gateway cities, then take trains around the region before returning to the gateway city to fly home.

"Long-distance train trips have enormous appeal among prospective Amtrak passengers, but many people lack the vacation time for a cross-country train trip both ways," said Timothy P. Gardner, Amtrak's vice president of passenger marketing. "The Amtrak Air-Rail travel plan will enable travelers to take an extended train trip and then fly home."

In the past, people who rode the train one way and flew a plane the other way got stuck paying full one-way fares, but United and Amtrak said their rates on the new tickets will be roughly equivalent to a round-trip air fare.

Amtrak approached United about three years ago, after determining that United's "route system is a better match with our route system than the other airlines," Amtrak spokeswoman Sue Martin said.

At least one airline analyst doubted that the train-plane arrangement will amount to much more than a marketing gimmick.

"I don't think there's a big market for it," said Raymond E. Neidl of Dillon Read & Co. Inc. "It might generate some marginal revenues. I don't think it's going to be that significant."

A Washington-based group that promotes rail travel as environmentally sound was pleased to see the deal come together, although a spokesman was not familiar enough with travel marketing to predict how successful it might be.

"I think anything that encourages people to take the train on any part of their journey is desirable," said Scott Leonard, assistant director of the National Association of Railroad Passengers. Leonard said rail transportation is particularly important these days because it is energy-efficient and could cut the nation's demand for foreign crude oil.

Gardner predicted Amtrak will attract thousands of passengers through the deal. A United spokeswoman, Sara Dornacker, said the airline would not reveal how much extra business it hopes to pick up.