New York has one of the oldest and biggest subways in the world, and as it has expanded, the city has used almost every letter in the alphabet to name its new lines. Conspicuously absent? The P line. Probably for the exact reason you'd imagine.
There are a handful of letters that don't exist as train lines, and a fascinating post on Mental Floss, Hannah Keyser explains why certain letters have gotten shafted in the great subway naming lottery.
Retired: H, K, T, V, and W
For example, the H, K, T, V and W lines are all lines that were decommissioned. The MTA will often recycle these letters—for example, the H Train was dusted off and brought back into use after Hurricane Sandy to describe the interim shuttle that connected the portions of the A train that weren't knocked out in the storm.
Too Confusing: I, O, U, Y
Other letters, like I and O, just look too much like numbers to be user-friendly. It's hard to argue with that logic—imagine trying to explain to a lost tourist that the downtown Zero train just doesn't exist. Same goes for the U Train, which sounds like a self-help book, and the Y Train, which would easily lend itself ennui-laden commuter jokes.
The P Train Mystery
That leaves us with the P train. What happened? Several lines have almost been named P, only to be surreptitiously renamed at the last minute. Keyser leaves us to imagine the reasoning behind the decision on our own—maybe the MTA didn't want to name a train after something its trains so often reek of.
Go read the full, fascinating post here.