The 13-corded cable with connector of the UIC 568 standard is a standardized connection cable from the UIC, used for transmitting a variety of data and commands between a locomotive and passenger carriages. Examples of those commands and data is loudspeaker messages, train destination signs, and control of doors (locked/un-locked, open/close) and others.
Locomotives equipped according to this standard will usually have two sockets at each end of the locomotive, and carriages will usually also have one or two sockets. Loose cables are then used to establish connection between the locomotive and the carriages, as well as from carriage to carriage.
Attempts have been made to use this cable for a complete communications bus (Train Communication Network, TCN), but it was decided that backwards compatibility was important, so that un-converted carriages could still be used. This would require an 18-corded cable, which led to the development of the UIC 558-cable and connectors. 13-corded plugs can be connected to a 18-corded socket, and the signals from the 13-corded cable carried through. The remaining five cords/pin is used for the TCN-bus and side-sensitive door control.
Read more about UIC 568: History