Those mysterious signs with colored sliders that passengers often see by the tracks on the London Underground and at railway stations? They’re actually datum plates. As journalists are saying, these devices are really important for safety. They record exactly where the track is on the platform, bridge, or support. The red slider shows the current position at the time of installation, and the green one indicates the reference (design) position. Engineers use this data to detect and track displacement in good time, prevent trains from colliding with platforms, and ensure smooth movement on bends.
For years, London Underground passengers have seen small signs with sliders, but they didn’t know what they were for. It turns out these are datum plates, which are critical to train safety. Mirror is reporting this.
You can find these signs all over the place, like at Underground stations and on London’s Overground railway platforms, but a lot of passengers didn’t really get what they were for for a long time.
It’s a little sign near the tracks with numbers, letters, and a slider. That’s exactly what got one user curious, so they looked up its meaning online.
He asked the question in the London Underground community on Reddit, posting a photo of the sign from the station.
Then a bunch of reasons were given. One user wrote, “This is what’s known as a datum plate. I think it’s used to measure track height and alignment, to check these things during engineering works and safety inspections.”
Another person agreed, saying, “That’s right. The same ones are used on the National Rail network”.
Another commenter explained more: “Track workers use it to make sure the track is at the right level compared to the platform. As far as I understand, you’ve got to align the track and the buffer stop, and then everything should be good. If there’s a big enough deviation, it could be a problem. The train might scrape the platform. My dad put these in a long time ago — I remember we used to have a whole kitchen full of these plates at home.”
Another user added, “This is a track reference (datum) plate. Some have red sliders, some have green ones. Red indicates the current set position, and green indicates the reference position, i.e. the design position.”
Railway enthusiast Railway Mayhem on YouTube says these plates actually hold the “secret” to safe railway operations.
“The colored block is a key element. If it’s red, it shows where the track was when the plate was put in, which helps make sure the train is the right distance from the platform. If the block is green, it means the track is in the ideal position according to modern standards. Engineers take into account speed limits, slopes, and track curves when they adjust the track alignment to make sure trains can move safely and smoothly,” explained the railway enthusiast.
The Railway Shop website, which sells decommissioned plates like this one, says: “Datum plates are attached to structures (bridges, platforms, catenary supports, etc.) located alongside the track. The info on the plate shows where the track is relative to the structure, so any movement can be tracked.”