Holmes: The glow at sunset is probably a simple optical effect – the sun's light striking the old glass panes of the station's roof and reflecting off dust or old paint. It could also be the result of a chemical residue from the old gas lamps that still leach into the air, giving the platform a faint amber haze. As for the symbol, it might not be a cryptic clue at all; many abandoned sites have graffiti left by passing travelers. If the symbol is consistently there, though, it could be a marker left by a clandestine group or an old station sign that survived the demolition. Check the paint layers, and look for any documentation about the station’s history. That should give you a starting point.

Sounds like a plan, though I'd keep my camera out of the way and my notebook ready. I can sense something beneath the paint. Let's see if the symbol matches anything on the old blueprints. I'll bring the flash light, a brush, and some swatches—no surprises if it's all a coincidence. If it's truly a secret, I'll find it.

Holmes: I'll keep my senses sharp, watch the symbol, the light, and the echoes. See you there.