I agree that it looks impressive (you just have to catch a time where the train is mostly empty to be able to see end to end), but 15 cars is a bit exaggerated. The U-Bahn trains have 6 cars, the current S-Bahn EMUs have 4 cars (up to three of them can be coupled, but you can't pass from one unit to the next). But the new S-Bahn trains scheduled to be delivered from the end of 2028 will be 200 meters long and have 13 cars (Siemens is apparently not superstitious) where you can indeed walk (and look) from one end to the other: https://press.siemens.com/global/en/pressrelease/germanys-mo...