Apparently, the system of dual-doors for the platform and the train I see characteristic on the JFK SkyTrain is rapidly become the 'norm' for metros, instead of the NYC-subway style system where you can touch the train directly from the platform.
Many of the the NYC subway trains run multiple conductors, to make sure that everyone is clear of the trains, and that no one is 'subway surfing' by clinging to the edge of the train doors. Given that labor costs are one of the major drivers of transit service, reducing long-term costs by the installation of a labor-saving feature seems like a reasonable thing to do.
The only issue is door spacing. Without screen doors on the platforms, the train doors can open anyplace on the platform, and transit agencies can run any combination of any style of trains, in any sequence. With screen doors, the distance between doors has to become an industry standard.
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