Under platform indicators?

[ajaxjones]

I am wondering what these devices are that are under platforms and how they work? 

Its said that they are something to do with rail levelling, would be nice to know what it is that I am adding to the platform side!

 

[Guest]

I think they're called datum plates. The red slider should be in line with the top of the railhead. The plate is capable of showing distance from railhead in millimetres, location along platform (I think they're spaced every 10 metres), superelevation of track if on a curve etc.

The above is from memory (I remember asking a while back), and if it's wrong - apologies!

[mjkerr]

They are used to ensure track alignment, where it is critical due to structures within or near the gauge limits
At other locations such alignment is not essential, hence why you don't see them

Obviously they are clearly seen at platforms, but can also be seen on bridges and OHLE

Normally they are used when ballast and/or rail has to be replaced
A gauge is then placed on the alignment bar (which does not have to be level with the nearest rail and the plate details this where applicable as it may not have been possible the plate in such a location) and the rail nearest to the structure
The measurements on the board are then compared to those from the gauge used to ensure the rail meets it's requirements 
If there is a variation, normally a tamper is then used to compact the ballast, and this is usually enough to meet the stricter clearance at these locations

The other situation is for bridges and where the ballast cannot be readily moved
The gauge is typically used where the rail itself has been removed
Where there is a variation then shims are placed under the rail before final clamping into place, as the shims become compressed over time

[ajaxjones]

Thanks, found a place that also sell the real thing

 

http://www.terram.com/products/signs-and-markers/railway-datum-gauge-sign.html