Railway signalling
（重定向自Railroad signaling）
![The freight train (right) is waiting at a red signal while the passenger train (left) crosses its path at a junction.](/Images/godic/202502/06/Freightliner_66549_gives_way_to_a_FGW_HST2749.jpg")
![British lower-quadrant semaphore stop signal (absolute) with subsidiary arm (permissive) below](/Images/godic/202502/06/Rail-semaphore-signal-Dave-F2749.jpg")
![Railway infrastructure on the hill-country main line, Sri Lanka, including a gantry of semaphore signals](/Images/godic/202502/06/Gantry_of_Sri_Lanka_semaphore_signals,main_line,Sri_Lanka2749.JPG")
![Short signal blocks on the Toronto Transit Commission subway system. A train (not visible) has just passed the most distant, leftmost signal, and the two most distant signals are red (stop and stay aspect). The next closest signal is yellow (proceed with caution), and the nearest signal shows green (proceed).](/Images/godic/202502/06/TTC_signal_blocks_captioned2750.JPG")
Railway signalling is a system used to direct railway traffic and keep trains clear of each other at all times. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which make it difficult to quickly stop when encountering an obstacle. In the UK, the Regulation of Railways Act 1889 introduced a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the Armagh rail disaster in that year.