A flicker, huh? Sounds like a signal riding the rails, or maybe just a stray photon. Remember what Schneier says about the human eye—always hunting for patterns that aren't there. If it’s a data line, it’ll have a fingerprint; if not, it’ll fade before the next train. Keep your guard up, and your camera on. If you catch it, let me know—maybe we’ll decode the tunnel’s secret code together.