This was Alco's second attempt to build a replacement
251-engined switcher to replace the
539-engined
S-3 and
S-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, the
S-6 with power output raised by 100 horsepower (75 kW).