This comes up every few weeks on HN but nobody has ever offered any statistics that would suggest this is as good let alone better than just having the trains handle alignment automatically. It's a task humans are bad at and machines are good at, so just giving it to machines makes more sense, modulo unions.

London Underground hasn't had guards for decades at this point, and the Docklands Light Railway hasn't even had drivers (there is a member of staff who is trained to be able to drive it on every train, but they are usually doing other things) since its creation. If they're misaligning often enough for it to be possible for New York to be statistically better I haven't seen anything about it after repeatedly asking.

Actually what exactly is the member of staff doing on the DLR that is necessary, other than answering tourists' questions and putting a triangular key into a receptacle at every stop and then turning it? I have not been able to figure this out.

In the Netherlands, the NS has two types of trains that go between towns. Intercity and Sprinter. Sprinters have someone who will walk onto the platform at every stop, or failing that, lean out of the carriage, verify that no one is getting in, and then step into the train again to put the key into the receptacle and then turn it. Following that, the doors close. In contrast, there is no such person on Intercity trains; they do fine without. There may be a conductor who checks tickets. In comparison to the DLR, both Sprinter and Intercity trains have drivers.

Is there some requirement or function that I am missing that requires a dedicated member of staff to perform this key-turning ritual at every stop on the DLR and Sprinter, or is this simply to appease the unions?

It could be that Sprinters are meant to be more lenient towards people running to get on than Intercities, which might have a stricter schedule.

It's a GoA 3 system, so it isn't designed to be safe without a human staff member on every train. There are GoA 4 systems which do not need a human but the DLR isn't one, so while it would seem to operate normally if you just let passengers operate the doors - when anything goes wrong those passengers are in trouble because the system design assumes a trained member of staff is there to fix it and now there isn't.

That triangular key opens a panel by the front left seats of the train, which reveals a complete set of controls for manually driving the train which that member of staff is trained to use. If the GoA 3 system has given up when the train is just out somewhere random then "just get out" while technically possible since there's a walking route along the side at all times - is clearly not ideal even for able-bodied passengers, so in fact the member of staff will drive the train manually to a station unless obviously that's impossible somehow (e.g. terrorists blew up sections of track either side like a Hollywood movie).

Because humans are bad at driving trains, they aren't allowed to move at full speed, they can either let the GoA 3 automation oversee everything (e.g. it won't let them go anywhere it wouldn't be willing to go) at a reduced speed or when that's not useful they can switch off all automation and move at a crawl with no oversight.

Every morning the first train of the day on each route is driven in the first of those two modes, because overnight human maintenance teams sometimes manage to leave tools and equipment on the line and the automation doesn't know not to drive the train into a welding kit left on the track by some idiot who just discovered his wife is leaving him or whatever. So the human staff member's job is to drive the train (with the AI preventing them smashing it into other trains) while looking out the front window for problems.

That clarifies things. Thanks for the in-depth explanation.