Tesla SF127

User apparatus of the so-called railway selector system. It was developed in the 1930s and was used on railway to provide connection between railway stations. A two-wire line was installed over the length of the track and the user units would be connected to this line. It was possible to call any other user, make group calls, call all the other stations at once or call the controlling dispatcher. In case of emergency, it was possible to forcibly interrupt any ongoing call. However, the system was quite unreliable and required precise maintenance. The last selector lines were put out of service in the 1990s.