RhB signal


In this video:


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

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