The Chicago, Burlington & Quincy Railroad received twelve 2-10-4s, which were called "Colorados" on the CB&Q, from the Baldwin Locomotive Works in 1927 and Baldwin delivered another six in 1929.

These "Colorados" were designated Class M-4 and were assigned road numbers 6310 through 6327. They were built to have 31" dia. x 32" stroke cylinders, 64" dia. drivers, a 250 psi boiler pressure, a tractive effort of 90,000 lbs and a weight of 512,110 pounds.

All eighteen of the Class M-4 locomotive were rebuilt by the CB&Q. The cylinder diameter was reduced to 28" with a resultant change in tractive effort to 83,300 pounds. Several other improvements were made including substitution of lighter materals which reduced the total weight of each locomotive to 501,200 pounds. These changes allowed the rebuilt Class M-4a "Colorados" to achieve higher speeds.

There are no surviving CB&Q Colorados.