The Class M-3, 2-10-2 types were built by the Brooks Works in 1919, and followed the USRA standards for locomotive design. Their general appearance and construction details were similar to the Class O-4 Mikados, except that Southern valve gear was employed. The M-3 engines were assigned to the road as No. 6300-6309. They operated on a boiler pressure of 190 psi, weighed in at 293,000 lb. on the drivers, and exerted a tractive force of 73,000 lb.. Their cylinder dimensions were identical to the Class M-2 and M-2-A engines. The M-3 locomotives saw most of their service on Colorado & Southern tracks, as they were leased to this subsidiary by the parent Burlington. All of the engines were equipped with Worthington feedwater heaters except No. 6303 and 6308, which were fitted with Elesco systems. The USRA Santa Fe types performed notably during their 30 years of service on the road.