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Comment by potatolicious | Hacker News Reader

> " First of all, the main problem for New Yorkers is the lack of this data for all the other train lines (i.e A-Z)."

Unfortunately this is not going to be fixed any time soon. BMT/IND trains have a distributed command and control system. Multiple signal towers control train movement within their own sectors of the system - and each signal tower has no information about the rest of the system.

This was necessary in the early days, before electronics existed.

So to this day there is no centralized snapshot of the entire system state, unlike the numbered lines, and thus no ability to provide an API.

This is a great step forward. I look forward to the day we can have a big jumbotron with real time locations of trains.

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What legacy system could possibly prevent one from simply recording the position of every train every few seconds? This is not a particularly difficult problem for a billion-dollar business to solve.

One built in 1904.

The control systems of the MTA are purely electromechanical. It goes way beyond what one would normally call a "legacy system".

You can't record the position of trains because there is no way to snapshot the whole system. Only the signal tower local to that section of the system is aware of what's going on in its zone. This is a vestige of the design where there was no reasonable way to centralize that amount of signaling over such great distances.

To get a snapshot of the whole system you'd have to simultaneously snapshot the state of every signal tower in the system and find some way to collect this information centrally. This sounds reasonable until you realize that the control system, being 50 years pre-computer, is unaware of individual trains. It doesn't detect trains, only presence of at least one car on a section of track (detected by the third rail making contact with the train).

To ascertain which train is where you'd have to either have dispatchers identify the train by communicating with operators (hard to poll), or infer the identity of a train by monitoring state. Both are quite difficult when all of your data is spread all over the entirety of NYC in isolated distributed control centers.

The IRT trains spent billions centralizing all command and control into a single building in Manhattan, and thus snapshotting state is trivially easy. The BMT/IND segments of the system are substantially bigger, with more stations, than even that. A full modernization to electronic control is necessary, which will cost many billions.

So on the decentralized systems, how does one station know when to delay a currently-stopped train because there's a delayed train farther up the route? Do signal towers up ahead simply transmit to adjacent stations "Delayed train up ahead?"

Pretty much. A signal tower usually controls several stations, so delays up ahead are usually contained within the zone of control of a signal tower.

In cases where delays are more substantial and you need to hold trains further upstream, this is communicated manually between towers.

Much of the control here is not based on semantic data, but rather verbal. The actual control system itself isn't capable of much more than indicating which piece of track is occupied.

I would guess it does not know there is a delayed train on the route, but just that there is a train on the route.

It is fairly easy to hook up a "'low resistance between the rails aka 'wheels on the rails' aka 'there is a train here'" detector to a 'do not enter' signal a few hundred meters up, and to a 'drive slowly' signal some distance before that (http://en.wikipedia.org/wiki/Automatic_Block_Signal)

I thought the presence of the train on given section of track was actually detected by the trains wheels completing the circuit between the rails, not with the third rail.

I don't care about the signal towers or how old the system is. The point is that the technology for identifying the location and speed of a few hundred giant hunks of metal moving around on fixed tracks is trivial and does not need to interact with the old system (or humans) at all. Just tag each train with a few RFID's, and stick in a few hundred miles of wiring into the tunnels.

I mean, are you telling me for a 100 million dollars this couldn't be done?

Let's assume that there's a byzantine system in which two parallel and unconnected systems control both BMT/IND trains...at some point, each train communicates arrival and destination data to its respective system. Why can't both systems report to a single controller? At some point, this information is centralized (dear God I hope it is, especially for evaluating performance metrics)...I can't see how it would be hard to provide this digitized data format at real time.

For starters: there's no electronics involved. It's all relays and stuff.

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