Great Western Coffee Shop

Please bear with me whilst I try and describe this device!

Whilst waiting at some stations I have noticed that on the edge of the platform (often on the blockwork that comes out of the ground) there is a metal plate about 8x6 inches with a slider control on it.these often have two numbers and the word "off" printed on them. If anyone knows. What I am talking about what are they for?

I am obviously spending too much time at stations!

Do you mean on the vertical wall of the platform just by the track? And is it roughly at ttack level?

Yes that's it often more than one

It's not actually a switch but a measure of the cant of the rails usually in mm. It gives the PW staff a level to check that rails are at the correct height relative to each other.

We have them at Taplow.  I presumme the stations you've seen them at are on a curve or coming out of or going into a curve. At Taplow you can see the increase in cant to the apex of the curve then the levelling out as the line straightens.

Well there you go - I would never have guessed that! But in which case I have a further question...why would they be on the likes of P2 and p3 at Reading?

I think that they are most usually installed when clearances are especialy tight, either verticly or horizontaly.
In many cases it does not matter much if after track repairs or renewals, the rails are reinstated in a SLIGHTLY different position. Who cares, in most cases, if the gap between train and platform is 30mm more or less than before track repairs.

In some cases though it is more important, for example on routes used by both Eurostars and networkers, if the rails are replaced even slightly closer to the platform, then a Eurostar might scrape the platform edge.
If the rails were replaced even slightly further away, then the gap to be stepped across from a networker would be unacceptably wide, remembering that it is already generous.

Vertical clearance is also important, especialy if already tight.
I recall a mishap just outside Paddington when the safety valve atop the boiler of a steam locomotive struck the underside of a bridge. Enquires showed that work to the track had raised it slightly, thus reducing the clrearance under the bridge.

These Datam panels appear to be appearing on all Platforms, from observations of them being installed, the Red slider is at the height of the highest rail and is a Lazar reflector, and at best guess the various figures of 978 985 996 on different panels would I guess be the height of the Platform above Rail Level.

The red or green slider defines the 'design level of the nearest rail' (sometimes it is at the exact rail height, often it is a standard distance above, such as the 100mm shown on the picture I've linked to).   The main 'offset' dimension shown is the horizontal distance between the plate and the running edge of the nearest rail.  Track cant at the location is also recorded.  So none of the dimensions actually show the platform height, but if the plate happens to be attached to a platform then the information will allow the track position with respect to the platform edge to be determined and maintained.  The plates do not just appear on platforms though, but on any lineside structure where dimension to track is relevant to track maintenance.

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

In that example, a horizontal line extended out from the red marker would be 100mm above the nearest rail - obviously the other numbers are just a 0-8 sequence so not typical.

The purpose of colour coded sliders is that a red slider can be re-adjusted by the maintainers to follow a gradually changing track position if movement occurs but it remains in tolerance with respect to lineside equipment - so a red slider might be found on an OHLE mast or signal post some distance from the track. However a green plate means that track position must be maintained in the absolute position shown.  AIUI all modern installations alongside platforms should have green sliders.

A real life example and explanation of all the info is shown on railsigns.co.uk here:  http://www.railsigns.co.uk/info/other1/other1.html

Paul 

Ive just been catching up on some of my old posts and missed some of these replies so thanks to all.

I am now alot wiser and wait for the day a question on these comes up in a pub quiz...