For some scale: The shortest line in London (ignoring 2.5km Waterloo &City) is Victoria line with 21km. The longest line, U1, in Nuremberg is 18.5km and still

For some scale: The shortest line in London (ignoring 2.5km Waterloo &City) is Victoria line with 21km. The longest line, U1, in Nuremberg is 18.5km and still doesn't run automated. They started with U3, which is running 9.2km. Even with only two ready rooms maximum distance is 2.5km, which can be travelled in a few minutes in case of emergency (using a car and emergency signals it's less than 5 minutes, but of course getting underground and from nearest entry point to train will take longer)