New York City subways' ancient signaling systems keep trains going slow – but that's about to change

New York is the most populous city in the United States and features the largest metro system in the Americas. However, it's also an old system. Because of this, it uses a lot of old tech.

Most lines on the NYC subway use an old system called "fixed block signaling" to ensure trains don't crash into each other. This basically breaks up each section of track into sections ("blocks") that only one train can run on at a time, as determined by a circuit in the track itself. It requires blocks to have enough padding to ensure that a train has enough time to stop if needed.

While this is fine for safety, it also results in a lot of wasted space between trains running at max speed, which means less frequent trains. If a transit agency wanted to run more trains using fixed block signaling, they would have to design smaller blocks. But with shorter blocks, trains need to be able to stop faster to avoid collisions. As a result, they have to significantly reduce speeds. In NYC's case, extremely high demand means that the Metropolitan Transportation Authority (MTA) is inclined to run more frequent, but slower trains on many routes.

There are various kinds of block signaling that somewhat improve on the basic concept. You can read about them on Wikipedia. The important part is that fixed block signaling relies on the track for the signal.

However, the MTA is contracting a Danish rail technology firm, Nordic Signals Consulting, to implement a more modern signaling system called communications-based train control (CBTC) to the subways. This isn't new technology at all (it is very proven), but much American infrastructure is ridiculously outdated.

CBTC allows traincars to automatically communicate with each other without relying on track circuits. The trains are fitted with radio transponders that communicate with regularly spaced transponders on the tracks, and in turn a centralized control mechanism. This provides more accurate data about the position, speed, and minimum braking distance of a train in real time. This allows for higher speeds along a particular route without reducing frequency or safety. It also allows for more automatically operated trains, and better integration with modern digital systems in general.

This system can be called "moving block" communication because the "blocks" that a train occupies are calculated in real time according to the train's position, rather than being fixed according to detections from track circuits. (So technically a system can use CBTC and block signaling at the same time; the distinction is whether the blocks are fixed vs. moving.)

The other advantage of CBTC is that it requires fewer physical track-side instruments. This reduces maintenance costs considerably, as well as reducing delay times when maintenance happens.

It's possible for a transit agency to screw up a system like CBTC if it doesn't adjust its operational procedures to suit the new model. However, I think that will not happen here. Consulting with foreign experts is an example of "learning from other countries" that the US fails to do on a regular basis; the fact that New York is making an effort here is a good thing. And it's not like CBTC is entirely new to the NYC subway system, they're just making an effort to scale up the implementation.

To actually use CBTC, the MTA also has to buy new trains. When Andy Byford was running the MTA, he planned a 10–15 year implementation schedule at an estimated cost of ~$12 billion. That would put full CBTC implementation at around 2030–35. I'm not sure if I believe that, but it's better than their plans from last decade, which would take at least 40 years. If anyone, Byford knew what he was doing.

The only competing system would be a less well-understood "ultra-wideband" (UWB) radio-enabled train signaling, which the MTA apparently plans to test while it's upgrading fixed-block signaling systems to CBTC. Its main advantage is apparently that it's easier (cheaper) to install than traditional CBTC, and it also extends cell phone coverage underground much more effectively. UWB radio communication would replace CBTC transponders, but would otherwise work approximately the same way as a moving-block CBTC system, from what I know.

You can read more about the signaling systems in the New York metro on Wikipedia. Articles in this part of the encyclopedia have an extremely small authorship and therefore occasionally dubious neutrality, and are consistently outdated, but I think they are usually technically accurate.