termed European Train Control System (ETCS), is a functional specification that incorporates some of the former national standards and allows them to be fully interoperable with a few modifications. Cab signal types All cab signalling systems must have a continuous in-cab indication to inform the driver of track condition ahead; however, these fall into two main categories. Intermittent cab signals are updated at discrete points along the rail line and between these points the display will reflect information from the last update. Continuous cab signals receive a continuous flow of information about the state of the track ahead and can have the cab indication change at any time to reflect any updates. The majority of cab signalling systems, including those that use coded track circuits, are continuous. Intermittent The German Indusi and Dutch ATB-NG fall into this category. These and other such systems provide constant reminders to drivers of track conditions ahead, but are only updated at discrete points. This can lead to situations where the information displayed to the driver has become out of date. Intermittent cab signalling systems have functional overlap with many other train protection systems such as trip stops, but the distinction is that a driver or automatic operating system makes continuous reference to the last received update. Continuous Continuous systems have the added benefit of fail safe behaviour in the event a train stops receiving the continuous event relied upon by the cab signalling system. Early systems use the rails or loop conductors laid along the track to provide continuous communication between wayside signal systems and the train. These systems provided for the transmission of more information than was typically possible with contemporary intermittent systems and are what enabled the ability to display a miniature signal to the driver; hence the term, "cab signalling". Continuous systems are also more easily paired with Automatic Train Control technology, which can enforce speed restrictions based on information received through the signalling system, because continuous cab signals can change at any time to be more or less restrictive, providing for more efficient operation than intermittent ATC systems. Information transmission Cab signals require a means of transmitting information from wayside to train. There are a few main methods to accomplish this information transfer. Electric or magnetic This is popular for early intermittent systems that used the presence of a magnetic field or electric current to designate a hazardous condition. The