Talk:Cab signalling

Unsourced information that was removed
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).

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 and axles of the train close 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. 

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 closing of the track circuit by the train 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 receiving (on-board) equipment is now used, although the systems were originally completely electromechanical save for vacuum-tube amplifiers used in the on-board receiver units.

Cab signal systems have typically been overlaid on top of existing wayside block-based signal systems. Some cab signal systems have removed the wayside equipment 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.

Maximum speed without cab signals
The article currently claims that the maximum speed without cab signals is 80mph, but I had thought that the effective speed limits for passenger trains in the US is typically 79mph. This article ought to cite a source on the speed limit. JNW2 20:54, 2 December 2007 (UTC)

Automatic stop requirements
My understanding has been that exceeding the speed limit that's somewhere around 79 or 80mph requires that cab signals be present, but does not actually require automatic stop equipment (though once you have gone to the trouble of installing cab signals, there may be little point in not having the automatic stop equipment). JNW2 20:54, 2 December 2007 (UTC)

The "not exceeding 80mph" limit was imposed in 1947.
The article currently states "Cab signaling in the United States was driven by a 1922 ruling by the Interstate Commerce Commission that trains would be limited to a speed not exceeding 80mph without some form of automatic train stop." However, that limit was not actually imposed until 1947. In 1922, the Interstate Commerce Commission only required some form of automatic train control installed in one division.

One reference to the history available on the web is "Transportation Research Circular" Number E-C085, Januaray 2006 "Railroad Operational Safety", Status and Researchh Needs, published by the Transportation Research Board of the National Academies, available at http://www.trb.org/publications/circulars/ec085.pdf

On page 27: "The Signal Inspection Law of 1920 amended a 1910 act. With this amendment, on June 13, 1922, the ICC issued an order requiring 49 railroads to install by January 1, 1925, over one full passenger division, systems for a form of automatic train control."

In the next paragraph: "On June 17, 1947, the ICC issued an order that required lines with speeds of 80 mph or greater to use an automatic train stop (ATS), or a socalled ATC, or an ACS (70)." —Preceding unsigned comment added by GregGritton (talk • contribs) 23:11, 13 April 2008 (UTC)


 * As I understood it the 80mph limit was set to take effect in the future from some previous time as a further effort to spur ATS adoption. The railroads had many years of warning to implement the safety systems.  It was probably set in the 1930's and took effect after the war.Sturmovik (talk) 03:57, 19 April 2008 (UTC)

PRR Puse Code System Broken out
I broke out the PRR Pulse Code system into its own page. I removed the globalization tag as that part seemed to be the one at issue.

Victoria Line
There are a number of references in the article to the Victoria line system using a variation of "the power-frequency coded track circuit system developed by the Pennsylvania Railroad (PRR)" this information dated from 2008. The entire line has subsequently been resignalled. I don't know enough about signals to know if the new systems is also a variation of the above, but from this quote "Westinghouse Rail Systems is installing Westrace electronic interlockings with Distance-To-Go Radio to replace the legacy ATO and ATP systems on the Victoria Line, which operate via rail-borne coded signals. " from this Railway Gazette article seems to suggest that it is not and the article needs to be updated. Could someone more knowledgeable than I on matters of signalling please update this information. Thanks Mr Weeble  Talk Brit tv 17:44, 21 December 2012 (UTC)
 * This whole article needs to be updated. I'll fix it at that time as the MBTA doesn't use the system either, but does use a different form of coded track circuit so that's half wrong.  Sturmovik (talk) 04:29, 22 December 2012 (UTC)