I’ve had the same question work its way to the conscious part of my brain a few times lately, so I thought I’d share it here.
The question arises when I’m traveling through the underbelly of New York City on the subway at upwards of 30 mph (I know the speed because each subway train actually contains a speedometer, visible to any passenger riding in the exact middle of the train, right next to the conductor). I’ll be riding a local train as the express goes by (or sometimes, somewhat counter-intuitively, I’ll be on the express train as the local goes by…) when I happen to look out the window and see some of the passengers on the other train.
If the trains are moving at similar speeds this can actually be quite a fun little experience; for some reason, people are not as quick to avert their eyes from someone else who happens to be peering out the window.
Something about this brief connection with a fellow traveler makes me think: I am physically just a few feet from this person, and yet if I wanted to actually speak to him, or shake his hand, it could be nearly impossible. What if I happened to see someone I recognized? In a small percentage of cases, the local and the express are both stopping at the next station, so it might be as easy as walking across the platform.
But it could also be extremely difficult - what if, instead of two subway trains, you happened to be on a NJ Transit train moving side by side with an Acela leaving Penn Station, headed straight to Philadelphia?
So what’s the furthest you can be, in terms of effort, cost, time, and distance, from being able to shake someone’s hand, while still being just a few feet away?