The recent spat between Greta Thunberg and German rail left me thinking: can you buy a ticket in a German train and not get a seat? Is that only for local or also long haul trains?
There is no mandatory seat reservation in Germany. The only exception are cross border tickets for some international trains and night sleeper trains. So yes, you can end up with no seat, if the train has no more empty seats. However in 2017 they introduced a booking limitation for trains that are out of capacity in total. As also standing capacity is limited.
Thus, at least you can get to your destination even in a train without empty seats, contrary to the French system in TGVs, where you just can’t buy a ticket for a train that has no more seats and you have to take the next one with available seats.
Apart from the exception dunni mentions, buying a ticket and buying a reservation are completely independent of each other.
Only the cheap tickets (Sparpreis, Super-Sparpreis) have a Zugbindung, i.e., they are valid only for one particular train. Normal-price tickets allow you to take any train from starting point to destination, provided it does not have higher category than what you paid for, e.g., IC → ICE (this is within a certain period of time; a couple of days I think). That flexibility would be thwarted if you had to buy a reservation together with the ticket.
This provides for e.g. weekend-commuters who don’t know in advance when they can leave their work at Friday to buy a ticket either without a reservation (and take the risk of travelling in an overcrowded train) or with reservations for different trains (if they are willing to pay for multiple reservations).
Once, when I was buying a ticket at a DB counter, I asked for a seat next to others not yet reserved, because I knew I was going to travel with friends that had not made a reservation. The clerk pointed out that I could make reservations for my friends if I wanted, even though I was just buying one ticket.
Seat reservations in German trains are only a thing in long distance trains (marked EC/IC or ICE if serviced by DB but can also include the Austrian RJ, the French TGV, the Thalys or other European railways’ EC). Thus, for any local train trip your ticket comes with no guarantee of a seat.
Where there are seat reservations, these are typically detached from the fare ticket itself. Meaning if you buy a ticket from Munich to Nuremburg and reserve a seat in ICE 4711 but miss that train in Munich central station and end up being in ICE 4713, your fare ticket is still valid but you forfeited your reservation. Note however that most cheaper fares include Zugbindung (being bound to a train), meaning that with said cheap fare you must use the train you are booked on.
Standard price tickets (called Flexpreis) are not only not restricted to a specific train but also not restricted to a specific train type: you may always travel on lower trains. So if you have an ICE ticket from Munich to Augsburg but taking an IC or a local train is more convenient your ticket is valid (however, if your ticket is an IC ticket you cannot hop onto an ICE barring special circumstances such as a previous delay).
Wherever seat reservations can be made but are not mandatory, they cost EUR 4.50. So a reservation is clearly treated as a separate service. It is possible to have your reservation refunded if it is DB’s fault that you can’t take your reserved seat (e.g. if they use a different trainset that doesn’t have seat reservations marked).
Finally, there are a few trains that have mandatory seat reservation, i.e. you must reserve to travel on them and you must sit on your assigned seat (the ticket does not transfer to other trains of the same type). This used to include the ICE sprinter but now is mostly restricted to other railways’ services within Germany if that other railway uses a global pricing system (e.g. TGV, Thalys) and night trains if in a berth.
: with a couple of exceptions where local trains apparently allow commuters to reserve a Stammplatz (seat you take every time) meaning you would reserve seat 61 in car 6 for the 6:16 train into Munich Mondays to Fridays. Mostly not noticable and not available to the general traveller and I can only remember seeing this service offered by Alex, one of many private local train franchises.
: This is no longer true in first class where a reservation is included in your basic fare ticket. However, unless your basic fare ticket is subject to Zugbindung you again do not have to be on the train you reserved.
I've bought tickets in advance and even paid for a seat reservation, but because they had to substitute a different train with either fewer cars or radically different carriage/seat numbering, still been without a seat.
I've been able to get the 4.50 EUR reservation refunded, but not the train ticket itself - when you buy a DB ticket, you're paying for the right to be on the train.
This does not contribute to acceptance of trains as an alternative to driving or flying, so Ms. Thunberg was right to call DB out on this out publicly. Plenty of us less-well-known riders have.
A reservation might be voided if a train is canceled or has less capacity. If too many people are on a train you might need to leave it (in this case they typically prefer people with reservations then).
But while it is a common problem to be in a crowded train and reservations sometimes fails, it needs to be noted that it is also very common that you do get a seat, especially when reserve properly.
I'll add to other fine answers a point of principle:
left me thinking: can you buy a ticket in a German train and not get a seat?
Yes, as the rationale seems to be: The railway company will not keep you off the train and stuck waiting (sometimes overnight), just because some people aren't willing to sit on the floor. Many people - perhaps most people - don't like sitting on the floor, but prefer it to not taking the train; and when a seat is crucial for someone, then they pay for the seat reservation.
This is in no way unique to Germany.
I've travelled extensively for years through all of Germany. Overcrowding is common in both local and long-distance trains. In fact, one Intercity train that I took regularily was often so crowded that I was always afraid I wouldn't be able to get on board at all (always managed, but a few times I had a packed standing place right at the door).
Seat reservations are only for long-distance trains, and even there they aren't mandatory. You can buy a ticket without a seat reservation. In fact, many of the non-budget tickets are not for a specific train, but for a specific route. So you buy a ticket from, say, Hamburg to Munich on the ICE train - and you can take any ICE within the period you booked for. Especially when I travelled on business, I usually had such flexible tickets as you never know if a meeting runs longer or the trip is cut short.
Also keep in mind that the german train company ("Deutsche Bahn", DB) has been privatized and has since aimed to be "profitable" - mostly by reducing costs. Many trains are planned badly, and even on the trains that are always overcrowded, they don't add cars. From the perspective of a profit-driven company, crowded trains are good.