Editing Cab signalling (section)

== Cab display unit ==
[[File:Metro North Cab Sigal.jpg|thumb|CDU used on [[Metro-North Railroad|Metro-North]] is integrated with the speedometer indicating the train speed, and the signals indicate the speed limit.]]
[[File:ERTMS DMI360.svg|The [[ETCS]] driver machine interface|thumb|left]]
The cab display unit (CDU), (also called a driver machine interface (DMI) in the [[ERTMS]] standard) is the interface between the train operator and the cab signalling system. Early CDU's displayed simple warning indications or representations of wayside railway signals. Later, many railways and rapid transit systems would dispense with miniature in-cab signals in favour of an indication of what speed the operator was permitted to travel at. Typically this was in conjunction with some sort of Automatic Train Control speed enforcement system where it becomes more important for operators to run their trains at specific speeds instead of using their judgement based on signal indications. One common innovation was to integrate the [[speedometer]] and cab signal display, superimposing or juxtaposing the allowed speed with the current speed. Digital cab signalling systems that make use of datagrams with "distance to target" information can use simple displays that simply inform the driver when they are approaching a speed penalty or have triggered a speed penalty or more complex ones that show a moving graph of the minimum braking curves permitted to reach the speed target.  

CDU's also inform the operator which, if any, mode the system might be in or if it is active at all. CDU's can also be integrated into the [[Dead-man's vigilance device|alertness system]], providing count-downs to the alertness penalty or a means by which to cancel the alarm.