If you've ever ridden the MTA's "Nostalgia Train" during the holidays (something admittedly hard to do since they run only on Sundays in December and are a catch-as-catch-can proposition), you may have noticed cars with covered passages allowing you to move freely from one part of the train to the next. These are known as open gangways, and were a common sight for much of the 20th century. But like many aspects of New York City life, the open gangway disappeared down the memory hole. Today, we have sliding doors that are usually locked or plastered with stickers discouraging their use, or both.
Meanwhile, subway ridership has skyrocketed, leading the MTA to consider re-introducing the open gangway concept for future orders of rolling stock (MTA parlance for subway cars). It turns out that such a design tweak would increase capacity for each train by eight to 10 percent. That's a significant addition. Newly released renderings show cars telescoping one into the next in a seemingly endless tunnel.
These renderings, however, aren't themselves new, exactly; they've been kicking around since 2013. More to the point, while the MTA seems to be including the open gangway concept in their 2015-2019 Capital Plan, it will only include ten such such cars out of a future order of 950—the idea being to see if New Yorkers cotton to the idea before proceeding further. If the notion proves to be popular, the MTA could conceivable retro-fit existing cars, or make their new-car orders for the 2020s all open gangway. Even if the MTA proceeded full-speed ahead, though, open gangways wouldn't be a system-wide feature for another half-century.
As a rule, the MTA is glacially slow when it comes to implementing change. (Just look how long they've dithered on replacing the current MetroCard with something that eliminates the hated swipe.) So don't count on being able to breeze between cars anytime soon.