A small sign featuring a series of numbers, letters, and a slider commonly appears beside the tracks on the London Underground, and many passengers have long wondered about its purpose. The mystery has now been solved thanks to a Reddit query that sparked explanations from railway experts.
What Is the Sign For?
One Reddit user posted a photo of the sign in the London Underground community, asking: "What's this for?" The responses revealed that it is a datum plate used for measuring height and track alignment. A commenter explained: "It's a Datum Plate, believe they are used for measuring height and track alignment, so it can be checked during engineering works and safety inspections."
Another person confirmed: "This is right on the money. This is the same as the ones on the National Rail network." A third elaborated: "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, there would be concern the train could collide with the platform."
Color Coding of Sliders
A fourth user pointed out: "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."
Railway enthusiast YouTube channel Railway Mayhem explained that datum plates hold the "secret" to keeping 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. But if it is a newer green block, that indicates the ideal track position according to new modern standards."
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, taking into account speed limits and track slope.
The Railway Shop website notes: "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."
