Savvy sub-way riders memorize which carriage to ride in and which door to exit that will guarantee them a quick exit up the stairs or a fast transfer to another line. I was never a public transport commuter until I spent a semester abroad in London in the early 1980s. During that time, I quickly realized there was a useful trick to optimizing your journey so that you never had to trudge behind a column of people up the stairs to get out or you missed your connection because you went down the wrong passage. It seemed obvious to me that with a little bit of observation, and by anticipating the placement of the exits and passages at my next station, that I could save considerable time. In a short while, I had it down to a science and to this day when riding the Underground, Path or Subway I still move about the departing station platform in order to make sure I get in the right car and so I can leave by the right door.
When Mrs PND and I first visited London together she couldn't grasp that whenever we traveled on the Underground I was always saying 'we can't stand here we need to go to the end of the platform' or 'I have to count the carriages to make sure we get on the right one' or words similar. Invariably, there are many stations in any system that are new to me (excepting the PATH) and thus if I end up at one of these unfamiliar stations I become a commuting victim just like everyone else.
Unlike me however; Jonathan Wegener and his sister Ashley thought that maybe "there should be an app for that" and have built an iPhone application that optimizes every NYC subway ride. They have created an easy and intuitive interface that enables you, for any combination of NYC stations, to find the proximity of carriages and doors to exit stairs and transfer points. It is a pretty neat app and represents yet another reason why the monthly NYTech Meetups can be so fun.
The Wegeners actually created this information by brute force . It didn't exist until the two of them spent 10 weeks riding the subway with clip boards in hand to document each subway stop. As far as I know, they weren't stopped by NYPD in the process. The application was introduced as the 'quintessential New York app' but I can see others copying the idea pretty rapidly.