The terms "inbound" and "outbound" are sometimes used to distinguish between directions of bus and train lines in public transit systems such as Boston's MBTA. They refer to directions of travel which go toward or away from some notional point of origin.
This makes at least some basic sense in most systems, since the general layout usually resembles a hub and spokes, or star topology, with the downtown area hosting a collection of more or less convenient transfer points among the lines (Boston, of course, being a prime example of such a hub), and routes that don't go to or from this area generally go to or from a single terminus... except the ones that don't. So maybe it isn't such a great idea after all, especially when you factor in the problem of what "outbound" means in a station in the center, where there's no "inbound" and several "outbounds", and some lines pass through the center so that there will be more than one outbound direction even on the same line.
Most transit systems seem to have solved this problem by giving up on inbound/outbound entirely, naming directions by their endpoints. Even Chicago's CTA refers to El trains as bound for either the Loop or a specific endpoint, not in or out. Boston, however, persists. In case you ever find yourself needing to know, here's how it's done:
- Commuter rail lines originate from either North Station or South Station and never pass through (except the special Red Sox train), so they're outbound when going away from Boston.
- Subway lines cross in a square whose corners are Park Street, Downtown Crossing, State Street, and Government Center, and away from this square is outbound. Within the square, inbound and outbound are not used. One quirk of this is that Bowdoin, the downtown terminus of the Blue Line, has an "inbound" direction.
- Bus routes don't have inbound or outbound; in fact, most bus stops don't even have route-specific information at all. Welcome to the joy that is the T.