The FL-9 was built by EMD specifically for the New Haven railroad. It was a dual mode loco based on the F9, that could run either on its diesel prime mover, or when entering New York City, would drop a "shoe" for electrical operation only. The prime mover was shut down. What made this economical was, it could make a commuter run from Boston to New York City with out a loco change, and then make a run on the non-electrified Berkshire Division the next. After the Penn Central merger, Metro North recieved all the FL-9's. Ten were saved, while the rest were scrapped. 2006 was the last year the FL9(or FL10, as Metro North designated them) being replaced by the GE's P32-42 Genesis. Naugatuck Valley Railroad has two of them, in McGuinnes paint. One is still being used by Metro North for back up power, and the rest were being scrapped.
There were no "B" units built.