If you have determined that you require seat reservations, it is always better to book them in advance to avoid disappointment. Premium European trains often sell out fast especially in peak months or at peak times of the day. You can however purchase seat reservations locally in Europe if you need to keep your schedule flexible. Reservations are normally available in various railway systems between 60 to 90 days in advance of your departure date.
See also: International Cross Border Trains.
Generally speaking, seat reservations for day trains and sleeper supplements for couchette accommodations are almost always required on International long distance trains, premium trains (high speed), all night trains and the majority of domestic long distance trains. For local trains and regional services, seat reservations are not normally required. If you are travelling with a Eurail, Interrail or any other Europan rail pass on these trains, simply board the desired train, grab a seat and relax.