Meaning of a black circle on yellow board

[MrJeeves]

Hey all,

Just passing through Watford Junction and spotted this out the window. Any ideas what it's for? Never seen it before.

I've no idea whether it's a "proper sign" or just something that's been put up for some reason.

 

[_toommm_]

I wondered this too a few months ago:

What are these? (not datum plates)

On the south WCML today, I’ve noticed a lot of these plates on the OHLE masts. Some yellow, some white. I can’t tell if it’s a cutout, or a black circle. What do they indicate please?

www.railforums.co.uk

 

[Lucy1501]

Never seen or heard of one myself before, but there’s an entry in the Rail Signs website, which is hugely helpful on matters like this! I believe it’s a sighting distance board, see the following link: https://www.railsigns.uk/sect28page2.html.

 

[MrJeeves]

Ahh fab! Thank you! That's solved one mystery for today

 

[Bald Rick]

They were put up around the time I started on the railway, and where I started, and this is the first time I’ve seen An explanation!

 

[The Chimaera]

Lookouts needed to be able to see two of these boards from their position, this ensured that they could see far enough along the line to give adequate warning time to allow staff to get to a position of safety once an approaching train was sighted.