What are the small sliders on the edge of the platforms at train stations.?

Question

they usually have some numbers next to the slider, are there are several of them along the lower part of the platfom by the tracks

just curious to know what they are used for.

Answer 1

Ok, i'm sitting on a train right now looking at them. On the line into London Paddington they're all level with the track. I'm guessing that the slider part is an exact distance from the top surface of the platform. The track engineers can then align the track height to them. This results in the train's door step height  being perfectly equally along the station.