http://www.wheelsotime.com/hd-21-dozer-n-scale/The Prototype -
Allis-Chalmers produced the HD-21 from 1954 to 1969; and to 1975 with a change in body style and specs as HD-21B. The HD-21 model included seven different variations of design: different body styling, power ratings, weight, air-filter canister placement. The HD-21 was the first Allis-Chalmer's model to use it's own manufactured engine based on the Buda diesel and not on Detroit diesel truck engine. Equipped with a 6 cylinder supercharged engine producing between 204 fwhp (1954 models) to 273 fwhp for 1963 model. They weighed in at 44,000 lbs (1954) to 57,100 lbs (1969 model) without any blade attachment. This means a 70-ton railroad flat car can carry a maximum of 2-3 tractors, depending on loading space and any additional attachments.
