Happy Thanksgiving!

Last week’s post before the holiday introduced subway signals, and today continues on the topic (make sure you’ve read the prior post before proceeeding).

Think about the red signal. Often indicating “stop,” it tells trains to, well, stop. But theoretically, with many caveats, the only problem with passing a normal red signal is hitting into the train ahead. For this reason, many subway systems allow operators to continue beyond a red signal at a “restricted” speed, typically at a speed allowing them to stop within half of their line of sight and no more than 15mph.

Where does this not apply? Switches.

Aside from the obvious concern of a switch sending a train in a different direction than desired, crossing a misaligned switch can often derail a train. In these cases, subway systems often use a super red aspect, consisting of a red light above another red light, to mean “stop and stay.” Crossing one of those signals at any time is a major no-no.

Since the signals adjacent to switches are so critical, they’re known as home signals. They differ from the signals between home signals, known as automatic signals, since their status is automatically determined by the position of trains ahead and the next home signal.

What other purpose does a home signal serve? They tell subway operators of the direction in which the tracks are aligned. Some systems, including the subways in NY and Toronto, treat the top part as a normal signal and the bottom portion as a direction indicator: green for the normal track and yellow for the diverging track. Elsewhere, such as on the PATH train, both signals are treated normally, with the top signal used when one is going on the normal track and the bottom when on the diverging track.

These schemes give rise to two different methods: the former always has a non-red aspect below a non-red aspect, whereas the latter always has at least one red aspect.

Nonetheless, armed with this knowledge, take a look out of the front of your subway train and see if you can begin understanding the signals. It also makes it clear why signal issues are a big deal:without the ability to effectively communicate information through signals, exceptional care must be undertaken to ensure safety at the expense of time and efficient operation.