Are they always necessary? Sometimes, especially in high summer season, but often you can wing it without one and will find unreserved seats to claim once you board a train. As the number of travelers continues to climb with the temps, reservations do become more and more helpful to have, espcially when you have a tight itinerary packed with specific destinations. Here in Germany right now it is a bit of a crap shoot.
Yesterday we tried to get reservations at the last minute for our trip from Essen to Berlin, and were told we couldn’t get any. We probably could have boarded the train anyway, and just took our chances on finding unreserved seats, but they would not be together or would only be in the smoking car. If those things don’t bother you, chances are you will always be able to find something without a reservation, especially if you are one person traveling alone.
I’ve also tried to book some reservations online about a week or so in advance but often get a message like this from DB or another train booking website: “Unfortunately there is no online booking possible for the requested connection. This is due to one of the following reasons. Neither an OnlineTicket nor a seat reservation can be offered. The deadline for booking the requested journey has passed.” But nine times out of ten if you show up at a ticket counter, you should be able to book a reservation, even up to several minutes before the departure time.