This was Alco's second attempt to build a replacement251-engined switcher to replace the539-enginedS-3andS-4.Only seven were sold. The demonstrator unit was ALCO specification DL421 and the six production units were ALCO specification DL421A. Alco fitted the 251A engine in its replacement, theS-6with power output raised by 100 horsepower (75 kW).