(High speed) trains may be the most comfortable mode of transport between major European cities. While some low cost airlines offer cheaper deals, remember that international high speed lines connect city centers, rather than airports that are usually located outside of the city. Also, trains do not require you to be present one hour before departure and can be part of the holiday experience: they allow you to enjoy the landscape, meet new people, have cup of coffee in the board restaurant or bring along a good bottle of wine.
Remember that the cheapest tickets are often sold out early and that reservations are generally possible 3 (normal) to 6 (CityNightLine) months in advance. Bookings can be made via NS Hispeed (Dutch railways) or its German and Belgian counterparts.