Berlin also has thosse door-in-the-front trains.
This is how our subway trains look like:
This is the newest model, particulary made for Düsseldorf. In former years they sometimes connected two trains for higher capacity in the subway system.
The reason we have this solution, is that as a wannabe-fancy state capital we need special manufactured trains which are more expensive than buying existing ones.
Another reason is that those trains are tram/subway hybrids, and our streets are crowded and narrow like crap, so longer trains would make no sense.