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 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 pounds on drivers, and exerted a tractive force of 73,800 pounds. Their cylinder dimensions (30X32) were identical the the class M-2 and M-2a engines. The M-3 locomotives saw most of their service on Colorado and 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. A comment prompted the following paragraph:

During World War One the production of locomotives was taken over by the United States Railroad Adminstration amid much controversy by the railroads. The goal was to rationalize production and conserve materials. Although the war lasted from 1914 to 1918, the actual USRA production began in mid-1918, four months before the end of the war. The reason for the U.S. on the tender was that locomotives were built in batches under USRA design specifications, then allocated to railroads based on need. Of the 175 USRA Heavy 2-10-2's built between 1918 and 1920 (end of USRA's reign), ten were allocated to the CB&Q (#6300-6309). Since the builder had no way of knowing which railroad would receive the locomotives built, all 175 received U.S. on the tenders, and upon allocation by the USRA, were finished with road name and number before delivery, as seen in the photograph.

The following is from Hol Wagner: It's my understanding that the NC&StL canceled their order while the locomotives were being built and the USRA looked for another road that would take them. The Q took them since it couldn't get 2-10-2s of its own design (though the USRA did allow it and the FW&D to purchase more Pacifics of Q design, thus the second order for S-3s, 2960-2974, in 1918 and for FW&D F-3A1's 551-555 in 1920). The USRA heavy 2-10-2s were a successful design, but the Burlington didn't like them. Their 63" drivers (3" larger than those of the M-2s) made them slippery on even the mildest grades and they had trouble starting long coal trains on the Beardstown Division where they were based. So as soon as the first batch of 10 M-4s arrived to replace them in 1927, the Q sent them west to the C&S, which badly needed additional heavy power and had no choice but to accept them. The Q logically believed that because the C&S already had five USRA heavy 2-10-2s of its own, the entire group of 15 could more efficiently be grouped together, maintained at the C&S Denver roundhouse and shopped at the four-year-old Joint Shops in Denver. And because the C&S couldn't afford to purchase the 10 M-3s, the Q exacted a daily rental fee from the C&S for their use -- a fee that over the 25+ years it was collected, would have more than paid for the purchase of the lot. So the C&S, which of course had much more of a roller coaster profile than did the Q, ended up with 15 virtually identical locomotives (yes, they were outfitted with two different feedwater heating systems), but of two different classes: E-5B for C&S 905-909 ad M-3 for Q 6300-6309. Like the Q, the C&S preferred the Burlington design 2-10-2s to the USRA design, but had no choice but to put them to the best possible use over its lines.