Want to take Amtrak this summer? I suggest you book now -- unless you were smart enough to have booked this summer's train trip last summer.

Unlike the confounding price fluctuations of air travel, the cost of tickets on any particular Amtrak train goes up as seats fill. The first in gets the lowest price.

I just scheduled my annual Thanksgiving trip via Amtrak to New Orleans, thinking I was early. The price had risen 25 percent over last year.

"You're bumping up against demand," explained Amtrak spokesman Marc Magliari. Ticket prices have gone up only about 4 percent, but increased ridership has tightened the market, he said.

Amtrak carried a record 30.2 million passengers during the 2011 fiscal year, which ended Sept. 30. That's an uptick of more than 5 percent from 2010. Magliari chalks the growing popularity to rising gas prices. "Driving is our main competition."

He suspects that some people booked tickets for this summer as a hedge against sky-high gas prices. There's little risk in that, even if those people decide to drive, because Amtrak does not charge a change fee and offers a full refund, unless a paper ticket was issued. Most passengers pick up their ticket at the station on the day of travel. Amtrak is moving toward e-ticketing, which will eliminate paper tickets altogether.

Travelers can book about 11 months ahead, a good idea -- especially for travel during busy times such as summer and the holidays-- given there's no downside. As for me, shortly after my return from New Orleans in November, I'll book my 2013 trip.

Send your questions or tips to travel editor Kerri Westenberg at travel@startribune.com, and follow her on twitter @kerriwestenberg.