This caught my eye:
I didn't look closely at it at the time, but having picked it out as a contender for blue, I started thinking about it. From the royal cypher GR it must date back to King George V (reigned 1910 - 1936) but the building it is in was built in 1995.
On the other hand it must be intended for use as otherwise the slot would have been blocked off to prevent rubbish being put in. And anyway why blue? Post boxes here are bright red. I e-mailed the railway but no reply......
This afternoon I had a bright idea. Why not try to enlarge the central panel which would have shown collection times plus the time of the next collection. And this is what I saw:
It is for children to post their letters to Thomas the Tank Engine!
This railway claims a particular association with Thomas as it is very near where the Rev. Awdry lived.
The other notable thing is that it isn't British gauge - it is continental (aka Berne) and it runs a selection of mainland European engines and carriages. http://www.nvr.org.uk/