The object itself is a balise, or Eurobalise; I think that one's by made by Siemens. The mounting beam is a separate product made by Pandrol; it avoids having to drill holes through concrete sleepers. Basically it tells the train where it is. Even in its basic form, with no connections to it, it can still transfer various other bits of data defined for ERTMS, but some only give ID and position.

They are used in ETCS, since the concept of a "movement authority" relies on the on-board control box knowing where the train is. You only need to add an odometer to provide positon between balises, corrected for drift at each balise, for a basic ETCS implementation.

Since the basic function of the balise is to say "you are here" with an ID code, there is nothing to stop you using them for other things just by programming the on-board systems. If you don't have ETCS it's particularly easy, since compatibility isn't an issue. Examples are train control functions like train stopping positon, station ID, selective door opening. But even a quite different design of object in the track (and even if not yellow!) can still be called a "balise" since that's just the French word for a lineside positon marker.

 

I think you were correctly informed Graham. The ones I was thinking about look like this. We have a similar one beside our track at Princes Risborough.

 

Really interesting all the various suggestions for No. 5 - it was advertised to me as European Train Control System (ETCS) tracked box - that system that's used on the Shrewsbury to Aberystwyth and Pwhelli lines to avoid trains colliding without physical signals.   But I suspect that in that box you could put all sorts of things!

 
5: Has the look of a remote track-circuit dropper. During a possession, it allows a PICOP to block a section of track by shorting out the track-circuit by remote control; thus not having to place themselves in danger by being on potentially live track and effectively to be in multiple places at once. Safer and more efficient.

 
1. Looks like braille instructions for blind train drivers

 
I would like to revisit a couple of others once the 24 hour rule has run its course.

 
3: Looks like a re-railing ramp

 
They are all trip hazards. 

 
1. reminds me a little of my Hornby Railway from childhood which included (in much more simple form) a carriage decoupler.

 
Sadly, I think that some will assume that 1 is somewhere you can stand to take a selfie, or ask your friend to stand while you take their picture.....

 
1. - I can see what it does, but don't know its name (or if it's used in this country). The shaped pieces rotate upwards to sit on the rails, and act as chocks or wheel blocks. So an alternative to trap points, perhaps, though I doubt these ones could stop a train of significant mass. (But as there is visibly something else built into the track off to the right, it's probably only a back-up system.)

 
5 Balise transponder for temporary speed restrictions

 
No 2.  Is a hydraulic rail stretcher used with continuously  welded rail in order to adjust the gap between sections prior to being Thermite welded.

 
4 is a crossing gauge or track gauge to measure the difference in level between the two rails at a single point.  This allows the cant to be accurately measured. Also important is the change in cant over  distance - the twist which must be kept within certain limits.

 
3 looks like a protection device relating to third rail.  Either to detect a train which has something low-hanging that would make unintended contact with the third rail section that it is approaching (1000 A at 750 V is not good if you're not expecting it), or to detect a train whose third rail shoe hasn't been raised when it leaves a third rail section.

 
5 - Axle counter

 
What are these?

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