The Alco S4 was distinguished from the S2 by the trucks, and from the S3 by the large radiator intake grills. The S2 rode on Blunt trucks, and the S4 had AAR Type A trucks. The S3 ( and S1 ) had a shorter radiator ( taller than it was wide ), and a straight pipe-like exhaust stack, indicating that the S1 and S3 were not turbocharged. It had a model 539 six cylinder McIntosh & Seymour diesel engine. The 5 in the diesel model number indicates 12½ X 13 inch cylinders and the 39 denotes the first year the diesel was fired up on the test stand.