Talk:Cab signalling
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The main purpose of a signal system is to enforce a safe separation between trains and to enforce speed limits. The cab signal system is an improvement over the wayside signal system, where visual signals beside or above the right-of-way govern the movement of trains, without any means of enforcing the signal automatically. While early cab signal systems only repeat the displayed signal, all modern systems have an enforcement component which can automatically bring a train to a stop. The first such systems were installed in the 1920s in the US, and later in the Netherlands in the 1940s. High-speed trains such as those in Japan, France, and Germany (LZB) use such systems in principle, though they may be mutually incompatible in practice. Therefore European rail authorities plan to introduce a standardized system, the European Train Control System (ETCS).
[edit] Cab signal systems in the US
In the US, there are many cab signalling systems, and at least one system must be operational for a train to exceed 79 MPH. One of the earliest systems was the Pennsylvania Railroad's system, which does not use transponders. This system uses the track circuit to transmit a 100 Hz electrical signal along the rails. The wheels of the train shunts the track circuit, and a pair of inductive pickups mounted 2 to 3 inches above the rail at the front of the train detects the 100 Hz carrier signal. The carrier signal is pulsed on and off at varying times per minute, allowing for four different codes. The four codes allow four speeds: track speed, 45 MPH, 30 MPH, and 20 MPH. This is more sophisticated than a train stop system. An improved version uses a second carrier frequency at 250 Hz to provide 9 signal aspects. [1]
Because the cab signal aspects are transmitted over the same track circuit that is used to detect the presence of a train, the circuits must be engineered for positive train detection and reliable transmission of the signal aspect to the cab. In the PRR system, the shunting of the track circuit by the train's wheels means that any trains following it will not detect any signal, resulting in the most restrictive aspect to be shown in the cab and on the wayside signal.
Power frequency cab signal track circuits use one or a combination of carrier frequencies at 60, 91.66, 100, 200, or 250 Hertz for train detection and communications. The frequency is chosen to avoid false vacancy failures due to stray energy from extraneous forces. DC circuits can be employed in territories without DC traction propulsion. (91.66Hz was employed by the Pennsylvania_Railroad in an area where DC and AC electric traction propulsion were sharing the same negative return rails and even harmonics of the 25Hz AC propulsion were being created by interaction between the AC and DC return currents. A 22-pole 25 cycle Motor-generator will create 91.66Hz)
Once the train occupies the track section, the track circuit energy which is now travelling primarily though the front axle of the train, is then modulated at 50, 75, 120, 180, 270, or 420 pulses-per-minute to supply a cab signal aspect which is detected by the onboard Automatic Train Control system and displayed to the Engineer. Generally faster modulation speeds are associated with faster speed commands, since a mechanically slowed "coding" (modulating) device will not result than an unsafely higher speed command being read by the train.
Solid state code transmitting (wayside) and recieveing (carbourne) equipment is now used, although the systems were originally completely elecromechanical save for tube amlifiers used in the carbourne reciever units.
Cab signal systems have typically been overlaid on top of existing wayside block-based signal systems. Some cab signal systems have removed the waysides so that only the cab signals are used, except at interlockings. Other systems use wayside transponders to augment the cab signal system, such as Amtrak's ACSES, which is in use from Boston to New Haven and on other high-speed sections on the Northeast Corridor, and NJ Transit's ASES.