How do railway signals work?
A good overview but a bit short on details. Some crucial things that got omitted (and are generally valid across Central Europe - no idea about the US, India and other places notorious for railway accidents):<p>- signals, both &quot;old&quot; mechanical and modern filament&#x2F;LED light based, are continuously monitored for their status. If a wire breaks (for mechanical signals) or the current is abnormal, a fault is detected by the interlock control and there are rigid procedures on how to operate safely under such conditions.<p>- Signals have some sort of direct connection to a railside element communicating the status of the signal (go&#x2F;slow&#x2F;stop) to the locomotive. In the case of the most common used system PZB [1], it&#x27;s a box containing a tuned inductor circuit, with the resonance frequency changing according to the status of the signal by connecting or disconnecting a capacitor (which can also be done by a mechanical signal). The train drives over these boxes and the locomotive&#x27;s antenna detects driving over a tuned or mistuned box. The French use a system applying a low voltage to a third, short center rail that gets picked up by a brush [2] or, for high-speed rails, a low-frequency FM carrier signal transmitted over the rails [3].<p>- railway sections are sub-divided into &quot;blocks&quot;, spaced at least a full brake distance from full speed apart. Signals signal to conductors what to do, and axle counters or DC monitors verify a block of rail is free of trains. The purpose of all that effort is to make sure that a) even in the worst case of a conductor missing a red light at full speed, the safety system has enough time to stop the train before it crashes into the train ahead and b) should a train separate and someone made an error in linking the central brake pipe (leading to the front part not braking automatically), the axle counter will detect that less axles came out of the block than came into it. It has the obvious disadvantage that a lot of capacity is wasted by the requirement to have one entirely empty block between trains; as a consequence systems like ETCS or LZB got developed that monitor position, speed and condition of trains and provide a virtual &quot;moving block&quot; leading to better utilization. In the long term, ETCS will lead to a complete elimination of track-side signals and eventually pave the way towards fully automated driving - but that&#x27;s <i>many decades</i> away.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Punktförmige_Zugbeeinflussung" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Punktförmige_Zugbeeinflus...</a><p>[2] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Crocodile_(train_protection_system)" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Crocodile_(train_protection_sy...</a><p>[3] <a href="https:&#x2F;&#x2F;de.wikipedia.org&#x2F;wiki&#x2F;Transmission_Voie-Machine" rel="nofollow">https:&#x2F;&#x2F;de.wikipedia.org&#x2F;wiki&#x2F;Transmission_Voie-Machine</a>