They were three private companies that built subway networks in New York City. One by one, they sold out to the city. The distinctions are now mainly of historical interest. The three letter abbreviations are obsolete, I don't think they are used in any official settings, many people who have come to New York in, say, the last 20 years probably don't even recognize them.

The song could be dated by the 75 cent fare, probably to some time in the 1980s. I think it's $2.75 now.