[mithaler]

I happen to live on the 7 train, so I know a fair bit about the difficulties of installing it there.

The 7 train is still controlled entirely manually, with the old block-based signalling mechanism that can only be aware of the location of a train in very vague terms. It's currently having CBTC (Communications-Based Train Control)[0] installed, which will give dispatchers much higher resolution information about the location of every train, and go a very long way towards automating the entire system. CBTC installation is scheduled to be completed in 2016, and it's the reason why there are currently so many service disruptions afflicting it during weekends (it actually just began a series of 13 consecutive weekends during which it will not go into Manhattan). If I recall correctly from the posters, the total budget of the project is about $750 million.

If and when the line is fully automated someday, it will become possible to create a reliable barrier mechanism (like what currently exists on the very small automated portions of the NYC subway, which is really just the JFK AirTrain). Until then, human operators can't reliably get the train cars to stop in the same place.

[0] http://en.wikipedia.org/wiki/Communications-based_train_cont...

[snogglethorpe]

> If and when the line is fully automated someday... Until then, human operators can't reliably get the train cars to stop in the same place.

Note that it certainly is possible to consistently do very accurate stops with only unassisted human operators; Japanese rail operators have done this as a matter of course, AFAIK using simple visual alignment aids, for as long as I can remember. [Traditionally it's done because people are lined up at the door locations (marked on the platforms), not for platform barriers.]

It's more a matter of operating culture and training than technology.

[Of course changing the latter may be easier in NYC!]