MBTA Green Line
The MBTA Green Line with its four lettered branches (B, C, D, E), is conspicuously missing an A branch. Ah, but there is a reason. Way back about 1967 when the lettering system was invented (previously the routes were numbered on maps and schedules, but not on the vehicles themselves), there was an A branch to Watertown. See the 1967 map below, featuring the A Line!
Here we find the letter “D” missing. Presently, there are terminals A, B, C and E, but no D. Well, it seems a few years back, Terminal D was merged into Terminal C. Plans were made to change Terminal E to Terminal D, but it would have caused some confusion.
How do you make the change? All the airlines would have to change their ticketing. And all the signs at the airport would need to change overnight, literally! And all the maps of the airport, and on-line maps, and Google maps, and MBTA maps, etc. And what if you fly out thinking you’d fly back into Terminal E, but you’re delayed and now it’s labeled Terminal D, but your friend trying to pick up at Terminal E can’t find it anywhere. So, I guess we’ll have an airport with a missing terminal for now.