London Underground users are only just realising the purpose of a small sign commonly seen next to the track at both underground and overground stations. The sign, which displays a series of numbers and letters alongside a coloured slider, has left many travellers puzzled for years.
What Is the Sign For?
One curious passenger recently took to the London Underground community on Reddit to ask: "What's this for?" alongside a photo of the sign at a Tube station. The question drew responses from knowledgeable individuals who explained that it is a datum plate used for measuring height and track alignment.
"Trackmen use it to ensure the track is at the right level to the platform," one user wrote. "If it's way off, there would be concern the train could collide with the platform." Another added that the slider must be set to level for everything to be safe.
How Datum Plates Work
According to railway enthusiasts, datum plates hold the "secret" to keeping railways running smoothly. The coloured block—red or green—indicates the track's position. Red shows where the track was when the plate was first installed, while green indicates the ideal position according to modern standards.
Engineers use the information from these plates, along with precise surveying tools, to make adjustments ensuring the track is perfectly aligned for safe and smooth train travel. The plates are fixed to structures near the track, such as bridges, platforms, and overhead line masts, and allow monitoring for any movement.
Some plates are no longer in use and are sold by retailers like The Railway Shop, which notes that the details on the plate specify the relative position of the track from the structure.
