The P-12-42, also known as the Speed Merchant, was a streamlined, 1,200 hp (890 kW) locomotive built between 1957–1958 by Fairbanks-Morse, specifically to operate on each end of the Talgo train produced by American Car and Foundry.

This model represented F-M's attempted entry into the lightweight locomotive market, but only four of the low-slung units were produced: the first pair was purchased by the New York, New Haven and Hartford Railroad for their John Quincy Adams train (New York, New York to Boston, Massachusetts), while the second pair went to the Boston and Maine Railroad for their Speed Merchant train (Boston, Massachusetts to Portland, Maine).

The P-12-42 was one of the first locomotives to have its prime mover configured to run at a constant speed, with traction generator output regulated solely by excitation.

F-M expected to sell many more units to the New Haven, and to spin the technological advance off into a line of passenger units, including a proposal that used a Train Master-style carbody, but with financial troubles preventing F-M from extending financing to customers,[citation needed] the cash-strapped New Haven ultimately opted to purchase a fleet of sixty EMD FL9s instead, as GM financing was available for the purchase.

The New Haven's Talgo train was sold to Ferrocarril de Langreo for service in Spain, in 1962, and their locomotives sat unused afterwards, until being scrapped in 1971.