It has happened more times than I can count. The doors slide open on a train car full of half-occupied pairs of seats. As the newcomer enters, we all set our features to maximum stoniness, flick the dimmer switch behind our eyes. New Guy picks his way awkwardly down the aisle. He is praying for an empty row to suddenly materialize, a resentment-free zone rich in the traveler’s most treasured resource: space. No luck. He comes closer, closer. Here he is. Here is his stupid bag being stowed against my feet. Here is his stupid arm on the armrest. Here...