What is Lock and Block?

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July 16, 2019, 1:00 PM

Lock and Block is a refinement of the Absolute Block system which is often used in areas with power signalling. (Although there is no direct connection with power signalling, the availability of electricity makes it possible.) It is especially relevant on double lines where entry of the train to the block section is controlled by taking off the last stop signal of the station in rear. With Lock and Block, the last stop signal cannot be taken off unless line clear has been received from the station in advance, and furthermore, once taken off the signal cannot be cleared a second time with the same line clear authority. In addition, the entry of the train into the block section replaces the last stop signal to danger. Lastly, line clear cannot be given for a second train until the first train has moved an adequate distance ahead of the last stop signal (and the signals have been put back to danger). This does not ensure that the entire train has passed, nor does it guard against train parting just beyond the signal, but does provide some additional safeguards over the basic token-based block operation. Track circuits or axle counters can be used to detect when the train has passed an adequate distance beyond the last stop signal. In early days – starting from the late 19th century, Sykes Lock and Block devices were commonly used to implement lock-and-block operation.

Source – IFRCA.org

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