Sorting people from rats, and other miracles of subway radar technology

Rendering of a Passenger Platform Safety System. (via PB-Consult)
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Can intrusion-detection technology, the sort of thing the M.T.A. says might help reduce subway deaths, tell the difference between a rat and a person?

The answer, as it turns out, is yes.

Nuremburg has a subway system, and two of the subway lines in that system are equipped with platform-end doors to keep people from descending to the tracks. The stations on those two lines also have track-bed radar beams in case riders find their way there by other means. 

Andreas May, who oversaw the installation of the system for a Germany company called PB-Consult, explained to me, in a series of emails, how the radars work.

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The radar system is made up of transmitters, installed beneath the subway platform edge, and radar receivers, installed at the opposite tunnel wall. 

Those transmitters emit a series of 24 GHz radar beams set 15 cm (about six inches) apart.

"To detect an object that might [be] dangerous for an train, 2 neighbouring beams have to be interrupted," Mays wrote. "If only one beam is interrupted we do not create an alarm. So we could avoid that small objects like bottles, rats or waste cause an alarm."

Why radar, versus other kinds of beams?

"We use the radar technology because it beams through thin objects like newspaper," said Mays.

Following a series of particularly gruesome subway deaths, the M.T.A. has begun exploring technologies that might help prevent them, like platform screen doors and some sort of track-level intrusion-detection system.

In Nuremburg, when an object interrupts two adjacent radar beams, the alarm system is triggered, and the following ensues.

"If the system detects an object it sends a signal to the [automatic train control] system," said Mays. "The ATC system stores the alarm, that means if a person goes back from a platform track onto the platform the alarm will remain. The ATC system sets the speed in the platform track to 0 km/h. Trains running into the direction of the affected platform track will be stopped 10 m in front of the platform track. Trains inside the platform track will be stopped immediately."

At the same time, the alarm is transmitted to the operations control center, where operators, using a video feed, can discern if anything is in fact on the tracks.

Of course, New York City does not have automated train control on its lines, but Mays said that's no problem.

"If we have an alarm of the [Platform Passenger Safety System] we switch on a red flashing light (emergency signal) in the tunnel to inform also train drivers," he wrote. "They have to brake the train. That means you can also use the PPSS in a conventional system and have a supervision of 24 hours."

Here are some slides from PB-Consult describing the system in greater detail: