In this video:

https://www.facebook.com/Bahnportraits02/videos/345311906477683/

you can see a single-lens signal at the tunnel mouth. I have only seen this before in Switzerland at a buffer stop (permanent red) which is clearly not the case here. I asked elsewhere and it was suggested it might be related to snow/ice doors inside the tunnel, but a signal that can only show red or dark doesn't sound fail-safe (does a dark aspect mean the bulb has failed or the line is clear)? I'm not aware that Switzerland has yet adopted the British/Italian/etc practice of using different coloured LEDs to display different signal aspects through a single aperture.

With a different audience here, can anyone advise on this signal's purpose? Also, is there an associated distant signal ("vorsignal")? Is it unique or are there others?

thanks in anticipation
Rob

Am 23.08.2020 um 16:36 schrieb Rob:

With a different audience here, can anyone advise on this signal's purpose?

It is a protection signal for the automatic door of the tunnel.
<https://www.flickr.com/photos/kecko/17133103218/in/photostream/>

A normal main signal must have a white border and means stop when dark. This signal should always be dark when a train passes by. It's there for exceptional situations, e.g. when a train gets stuck on the line or when there is work on the line and transports to the working site are needed.

Markus, Gürbetal