That's a very general question. If you're travelling strictly east coast or whatever, you can get by using trains. Amtrack runs routes between Boston, NYC, and DC... But if you were to attempt to go from east coast to west coast on a train, you could save yourself a lot of days (including food costs) by catching a flight. The problem is that airfare prices are on the rise, so you need to search for the cheapest fares between a number of different cities. If you can get by without checking bags, it's sometimes cheaper to book flights with 2 hops and get off at the first hop. (direct flights cost more on average)