
June 17th – At Spring Road station in Birmingham, I overheard to travellers discussing the white plates visible on the the edge of the opposite platform, down beside the track. They often baffle station users, and their usage is a bit obscure, really.
These are merely a datum survey marker for when the rails are replaced and the track relayed, which happens more often than one would imagine. Before the old track is moved, a surveyor uses a laser level or theodolite to measure the exact position of the rails, and their tilt angle if they have one. The rail height – plus any required offset – is set on the sliding knob, and is used as a datum for relaying new lines.
The legend proclaims this plate no 3; it’s 1105mm to the nearest rail, on the Up Main (UM) line. The cant (or tilt between the two rails to enable safer dynamic cornering of rolling stock) is 2mm.
The (just visible) +474 above the slider indicates that the level set is 474mm above the desired rail height (vertical offset) and a green knob says this is the level the rail should be at as it was designed, and may not currently be at that level. A red knob indicates the actual track position when the plate was installed.
Geometry like this is essential to rail engineers, who obsess over it. Maintaining correct geometry is of prime concern, prevents accidents and ensures trains fit under bridges, alongside platforms and don’t foul each other on bends.
You can often see these marker plates fixed to line side structures or electricity and signal masts.