Every day, millions of people rely on subways and trains to travel, and the signal system akin to more advanced traffic lights is responsible for ensuring their safe operation. Ever wonder how they work? You’re about to find out.

Most railroads operate by defining tracks into specific segments, known as “blocks.” Generally speaking, to prevent trains from hitting one another, only one train can occupy a block at a given time. Signals are how train operators know the status of blocks ahead.

A red signal is the most easy signal to understand: stop. This typically indicates the block ahead is occupied or can’t be entered since it’s a buffer for a train further down the line. Passing a red signal on most systems will often trigger emergency brakes.

A yellow signal often indicates “approach,” that is, the block ahead is clear but the following block is not (i.e. a red signal awaits at the end of the block).

A green signal often signifies a “clear” status, wherein the block is clear and the block after it is also clear.

So, if there’s a train four blocks ahead and a signal at the end of every block, one would encounter a clear signal, a clear signal, an approach signal, and a stop signal.

While this system works quite well for managing train movements, it has two huge limitations: block lengths and speed control. Fixed blocks have been great historically, but new technology known as CBTC (likely the topic of a future post) enables virtual blocks to govern train movements. However, this is still in the works.

What about speed limits? Since the signal system described above only accounts for preventing duplicate track occupancy, signal timers are used to enforce speeds. In NY, Toronto, and several other major North American markets, a white light is used to indicate that a given signal is only set to “approach” or “stop” due to a speed limit. As a result, the signal will upgrade to a more favorable signal (i.e. a green “clear” or a yellow “approach”) a given amount of time after one enters the applicable block (consistent with the posted speed). Traveling too fast causes the operator to hit the red signal, bringing the train to a halt.

There’s a ton more to discuss in this realm: What’s the issue with signal problems? How do signals physically detect and stop trains? Why are timers a major problem in NY, and what can be done? How are subway switches signaled? What are automatic signals? What’s CBTC? These questions will likely be answered in future posts.

For now, if you’re interested in skipping ahead to learning about more advanced signals, check out the NYC Subway website on their signal system.