If you were to pit the cost of airfare against the price of a rail ticket, which would win?

Ask Momondo.com, the Copenhagen-based aggregator, which searches 800 airline Web sites, 4,800 high-speed train routes and the largest online travel agents for the best price. Unlike similar sites, such as Kayak and SideStep, Momondo displays train fares with air prices on European searches, an especially important comparison in a destination where the two modes of travel are competitive.