Saturday, August 24, 2013

Revamping signal system for the subway

The other day I was watching an old episode of Extreme Engineering, Subways of America. One of the problems with scheduling is collision avoidance. In order to reduce the likelihood of collision, you space out the trains or keep them moving slowly. But that means the utilization of the track becomes low or you increase commuter time. Collision is a huge problem for subway because visibility in the underground tunnel is low, and once an accident happens, there are only very limited escape options. A smoking fire can quickly turn into a disaster. The signal system allows collision avoidance in a more predictable way, so that efficiency can be improved.

Although the episode focused more on NYC subway, I take the MBTA every day to work, and I often wished its efficiency could be improved. The signal systems of the mass transit lines, red and blue, have been recently revamped (I don't take the orange line so I don't know). The green line, on the other hand, is still using an old system. They basically stick a signal light with a sensor and a timer to the right of the track. When a train passes, the sensor detects the train, and turns the signal light red. After 10 seconds, the signal light turns yellow, and another minute the signal light turns green. (I have not actually timed the intervals. I just pulled them out from memory). Signal lights are spaced every some distance. When you hear MBTA announce signaling problem, one of the light bulbs probably just burned out or the light got stuck in green.

What a signal system boils down to is a way to estimate distance between two trains, so the conductor can adjust the speed of the train accordingly. But installing signal systems can be expensive particularly if it requires new wiring along the tunnel. It requires taking parts of the tunnel offline, and servicing is expensive. The ingenuity of the timed signal lights is that it's a decentralized, cost-effective solution that worked well 100 years ago when the pace of life was slower.

I propose a similarly decentralized signal system where each train gains the visibility of its distance from other trains without the need to install new wiring to the tunnel. The key idea lies in applying Broadband over Power Lines technology which converts the "third rail" or the power line into an ether where communication can take place. Similar to how GPS and NTP systems work, trains will broadcast time-stamped messages at a given interval, and by measuring the round-trip time of these messages, trains can estimate the distances between each other. Stationary beacons can be placed at fixed distances on the track so trains can also track their absolute position.

The stationary beacon also detect if a train is passing without the active broadcasting working. If so, it sends a warning to all trains. The train immediately behind the warning beacon will ask the beacon how long ago the no-signal train has passed and adjusts its speed accordingly, essentially reverting back to the old system. The system is successful if it would allow both old and new trains to work on the same track. A graceful upgrade option is also a graceful downgrade option.

Now done with the logistics, here is the technical part. Light travels about 30cm each nanosecond. This is called a "light-foot." Electric signal travels at similar speed. We can convert the round-trip time to a distance, but we're constrained by the accuracy and precision we can measure time. Computer processors run at 1GHz or more, so for the sake of discussion we can equate 1 nanosecond to 1 CPU cycle. It is generally possible to use off-the-shelf computers for this purpose, except off-the-shelf operating systems are not to the task. We need a dedicated real-time operating system that can achieve latency in just a handful of CPU cycles. Since main memory access incurs cycle penalty of 20 cycles or more, the system will run entirely off the CPU's L2 cache.

Using a general programmable CPU has the benefit that we can easily design and test the time-distance measurement protocol to make it safer, more efficient, and more robust. On the other hand, if this is programmed in hardware, then it would take many years. GPS has its root from 1940's, with initial deployment in its current form in the 1990's. It took GPS more than 20 years to reach current maturity. Although we're not launching satellites, the positioning system proposed here requires faster response time and more precise distance measurement, and will likely not be doable in another 10 years in hardware. I think we can already develop the technology in software in a matter of a few years.
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