Many years ago. I purchased a MTH DL 109. Three different rd. numbers were offered and a separate sale B unit. I purchased 1 New Haven A unit. Decided to look for another road number to run with it. These became desirable. The NH never had B units. It took a few years. Many for sale. But you were forced to buy the matching B at a higher than original MSRP price. Eventually I purchased both missing numbers.
I just never got. The ABA concept of marketing. Forcing you to buy all 3. Especially if you have a smaller layout and start adding a string of passenger cars. It takes up a good portion of the RR.