A few years later New York City purchased the two private companies and began to merge them. Unfortunately, due to different physical features and track infrastructure, the two lines had to be kept separated into two divisions, the A and B divisions. The A division is the number line trains such as 1,2,3, and 4. The B division is the letter line trains such as A,B,C, and D.[3]
After reading this far, the paper it sounds like it's going to be a history of the MTA.