What's this sign?

[304033]

Whilst waiting on the Aston Expressway (Distressway) earlier on, I noticed this sign. It appears to be next to signal SB4420 (was NS57).
Me being curious, what does the sign display? Thanks!

 

[zwk500]

Does it show anything or is just a sighting board (clean background to aid the driver in identifying the signal aspect)?

A street view Side-on image suggests it's a single sheet of material so unless the sign is blacked out for some reason I'd go with a Sighting board.

 

[43066]

It’s not a datum plate .

 

[Dave W]

Does it show anything or is just a sighting board (clean background to aid the driver in identifying the signal aspect)?

A street view Side-on image suggests it's a single sheet of material so unless the sign is blacked out for some reason I'd go with a Sighting board.

There's an angle from a minor road underneath the Expressway which shows the board directly behind the signal, so I fancy this is a pretty good guess.

The railway curves to the left under the road here, I wonder if it's to prevent confusion with the lane management lights?

 

[Sly Old Fox]

Nothing at all, it’s usually used just to block vision of other signals behind so the driver can just see the relevant signal.

 

[swt_passenger]

In addition to above, these are not only used to blank out non-applicable rail signals, but it’s also potentially there to prevent a driver seeing confusing road signals.

 

[mr_moo]

Train drivers view (forgive the dirty camera!) - The black backing board is positioned to stop car headlights from shining directly at the driver through the signal and thus potentially causing a mis-read and/or a driver distraction.

 

[304033]

Train drivers view (forgive the dirty camera!) - The black backing board is positioned to stop car headlights from shining directly at the driver through the signal and thus potentially causing a mis-read and/or a driver distraction.

View attachment 137265

That's really clever! Thank you