London Underground users only just realising what small sign next to track means

The purpose of a small sign that appears prominently beside London Underground and overground railway tracks has seemingly left many puzzled for years. Each one displays a series of numbers and letters alongside a slider, which prompted one person to seek answers online. "What's this for?" they asked in the London Underground community on Reddit alongside a snap taken of one of the signs at a Tube station. The question received responses from several knowledgeable individuals, with one explaining: "Its a Datum Plate, believe they are used for measuring height and track alignment, so it can be checked during engineering works and safety inspections." A second person agreed: "This is right on the money. This is the same as the ones on the National Rail network." While a third provided additional detail: "Trackmen use it to ensure the track is at the right level to the platform. My understanding is the track and the slider need to be set to level and everything is okay. "If it's way off the would be concern the train could collide with platform. My father fitted these many years ago, I remember having a kitchen full of them one upon a time." Then a fourth Reddit user highlighted: "Track datum plate some have red sliders, some are green sliders. "Red is the current installed position. Green is absolute so basically the designed position." On YouTube, railway enthusiast Railway Mayhem suggests datum plates hold the "secret" to keeping the railways running smoothly. "The coloured block is the key - if they have a red block, it shows where the track was when the plate was first installed, ensuring the train stays at the correct distance from the platform edge," they explained. "But if it is a newer green block, that indicates that the ideal track position according to new modern standards. "Taking into account the speed limit and the slope and incline of the track, engineers use the information from the plates along with precise surveying tools to make adjustments ensuring the track is perfectly aligned for safe and smooth train travel." Elsewhere, the Railway Shop, which sells such plates that are no longer in use, states on its website : "Datum plates are fixed to structures (bridges, platforms and overhead line masts, etc.) that are located near the track. "The details on the plate specify the relative position of the track from said structure, which can then be monitored for movement."