NYC subway secrets: Why no I, O, U, P, Y trains?

NEW YORK (FOX5NY.COM) - New York City doesn't have the busiest subway system in the world. That title actually belongs to Tokyo. And if you've ever taken a train here, you know we don't have the prettiest subway either. But we do have one that is easy to navigate. Trains either have numbers or letters. And as long as you know what direction you're headed, you're in.

The BMT, the IND, the IRT—if you think that all sounds like Greek to me, it is really quite simple. All three of those operating companies became part of the New York City subway system. So why is there no P train or no 8 train? So many questions, so little time. I found answers them at the New York Transit Museum in Downtown Brooklyn.

Associate Curator Jodi Shapiro is somewhat of a subway historian. She explained that the subway system used to be three different companies, each with its own nomenclature until the city unified the system in 1945.

The old BMT (Brooklyn–Manhattan Transit) system used numbers to order its trains, which prompted the IND (Independent) system to use letters. (The IRT [Interborough Rapid Transit] line was the city's first.)

As the MTA expanded, things changed. In 1979, the colors we see today were chosen. In 1985, all double-lettered trains were abolished. The KK morphed into a single K. Then in 1988, the K morphed into the A.

But why are some letters and numbers not represented at all? For instance, have you ever wondered why there's no P train?

Shapiro said a planned extension to the F line, officially known as the Culver line, was to be called the P line. But it never happened. The letters I and O were never used as designations because they look too similar to the numbers 1 and 0. The letters U and Y were never used because they sound like the words "you" and "why."

The moral of this story: there really is a method to the madness.