NYC Subway Signals: A Complete Guide (2012)

This article is somewhat out-of-date. The L & 7 trains were modernized to communications-based train control (CBTC) & automatic train operation (ATO) since 2006 & 2018 respectively. This allows for moving-block signaling, where a trains are separated by a distance, instead of fixed blocks of track. The legacy fixed-block signaling system is still retained as a fallback.

Very similar detailed site for the Chicago L. Here's the signals page, but a ton more to explore as well: https://www.chicago-l.org/operations/signals/signals.html

That logo though :) Tickled that the "Y" and only the "Y" reference NYC.

Lots of words for "chain in, rail out" as they say on Nauvis

A very interesting thing to read on a long NYC metro ride.