number of information points that may have been possible with older systems as well as finer grained signalling information. The British Automatic Train Protection was one example of this technology along with the more recent Dutch ATB-NG. Wireless Wireless cab signalling systems dispense with all track-based communications infrastructure and instead rely on fixed wireless transmitters to send trains signalling information. This method is most closely associated with communications-based train control. ETCS levels 2 and 3 make use of this system, as do a number of other cab signalling systems under development. Cab display unit 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 alertness system, providing count-downs to the alertness penalty or a means by which to cancel the alarm. Cab signalling systems in the United States Cab signalling in the United States was driven by a 1922 ruling by the Interstate Commerce Commission (ICC) that required 49 railways to install some form of automatic train control in one full passenger division by 1925. While several large railways, including the Santa Fe and New York Central, fulfilled the requirement by installing intermittent inductive train stop devices, the PRR saw an opportunity to improve operational efficiency and installed