B&M S-5 No. 864 working the yard at North Station, Boston, on September 4, 1965.

Number 864 is preserved at the Mad River & NKP RR Society in Bellevue, Ohio.

(Roger Puta, courtesy Marty Bernard, railfan 44, Public domain, via Wikimedia Commons)


Alco logo, red and white.


The Alco S-5 (DL 421A) was a diesel-electric switcher locomotive rated at 800 horsepower (600 kW), that rode on two-axle trucks, having a B-B wheel arrangement.

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).


Original owners

Railroad Quantity Road numbers Notes
Alco (demonstrator) 1 909 to Island Creek Fuel and Transportation Company No. 102
Boston and Maine Railroad 6 860–865


Type and origin
Power type: Diesel-electric
Builder: ALCO
Serial number: 80915–80921
Model: S-5 (Specification DL421 and DL421A)
Build date: June 1954
Total produced: 7
Configuration: AAR B-B
Gauge: 4 ft 8+1⁄2 in (1,435 mm)
Prime mover: Alco 251
Cylinders Straight: 6
Performance figures
Power output: 800 hp (600 kW)
Operators: Boston and Maine Railroad
Numbers: 860-865, 909
Locale: United States
Disposition: 1 preserved, remainder scrapped