carriage washer overspeed signs

[Bodie]

The other night I was travelling on the up sleeper to London.
Whilst stopped at Perth my carriage was opposite an electronic sign which scrolled with the message 'Wash - Ready'.

158740 and 158741 were not far from it. Then they set off and after they had passed the sign it now read 'Overspeed by 3.00mph'

Does anyone know more about how this system works?
Is it like one of those road signs that display your speed and either show a smiley face or sad face?

 

[t_star2001uk]

The one that is at Wembley LMD works on a pair of treadles attached to the rail which measure the vehicle speed....

 

[Cherry_Picker]

There is one at Tyseley (on the Spring Road side of the station) which also works like that.

 

[Clip]

They have to measure the speed because if they go too fast then the trains will no be cleaned properly and if too slow then they may suffer from brush burn which is where the rubber from the brushes actually collects on the side of the vehicles and over time can buld up to be a film on the side of the trains and be a pain to then clean off

 

[link1driver]

The one at Perth seems particularly sensitive! Think we all set it off last night. Trouble is, there is a downward gradient immediately after the washer towards P149 exit signal, although there are some who will try not to get the unit wet in case the rust increases