I'm starting to think that the requirements for getting this encounter is having less than 35 HP or something like that. I've spent the better part of 2 days trying to get this encounter while I have enough HP to survive it and it just doesn't ever show up. I've gotten it a dozen times when I have less than the 36-38 HP, so it always beats me up.
/me complains about bad rng.
Also, maybe something to do with attacking the ghosts? I seem to be getting the Echoes of the Past encounter again now, even though earlier today I was just getting attacked by them.
EDIT: I'm also getting a similar combat encounter in the flooded version of the sewers, which several other people also don't seem to be getting. I wonder why? It sucks running into these guys every third turn.
You take your time following the flooded trench. Even with the raw sewage, it's still nicer than a lot of Metros terminals.
But one moment you're appreciating the scenery… and the next the tiles under your feet are flowing like water, hurling you down into the trench. It's as vile as you'd expect, but gives you an excellent vantage point as three serpents of dull steel rise fluidly out of the filth.
The one nearest you crackles with energy you could mistake for electricity if it didn't cover the creature like a second skin. Flanking it are two more, looking down at you with grins of broken glass.
You've gained 1 energy of Submerged.
You've gained 20 energy of Vile Camouflage.
Fight 2 Rail Serpents and an Energized Serpent
FURTHER EDIT: Kinak says it's not terribly complicated as to why I'm getting this.
One more possible idea is that the Vile Camouflage is doing it. I currently have 198 turns of it. I like this idea because the subway is described as being mysteriously clean… so it's sentiently disposing of dirty stuff into the river. I'm more considering this idea because of flavour rather than any real data.