In the latter part of 1927, the Burlington received from the Baldwin Locomotive Works 12 locomotives of the 2-10-4 wheel arrangement, commonly called the Texas type after the design's origin on the Texas & Pacific in 1925. Numbered 6310-6321 and classed M-4, the Burlington engines (known on the Q as the Colorado types) were the largest and most powerful two-cylinder locomotives built up to that time. They were designed to replace 2-10-2's on southern Illinois coal drags over the Beardstown Division and in doing so to increase tonnage per train, obtain the greatest fuel economy possible and reduce the amount of overtime paid to crews on the slower 2-10-2's.

The boilers of the M-4's were of the radial stay, inverted wagon-top design, 92 inches in diameter at the first stay ring, and 104 inches across at the throat. The barrel contained 87 2½-inch firetubes and 222 3½-inch superheater tubes. The large firebox, carried above the trailing truck, was 150 inches long by 102 inches wide inside the sheets, giving it a grate area of 106.5 square feet. It was equipped with a combustion chamber and arch tubes. Southern Illinois bituminous coal was the locomotives intended fuel, and it was supplied to the firebox by a Duplex stoker. A low steam dome was centrally located on the boiler barrel, and was flanked fore and aft by large sand domes. Engines 6310-6315 were equipped with Elesco feedwater heaters, while engines 6316-6321 had Worthington BL heaters. All 12 locomotives were outfitted with Strombos quadruplex air whistles (actually horns) mounted atop the boiler just ahead of the front sand dome. These air horns were in vogue for a brief period of time during the late Twenties and were applied to a number of Burlington locomotives.

The head-end crew, including a number of Mechanical Department officials, posed with the still-new 6318 as the M-4 was recorded by a Baldwin photographer on one of its break-in runs in coal train service on the Beardstown Division late in 1927. Note the air horns behind the stack and white flag. Note too the single-chime whistle mounted on the large live steam line, just ahead of the rear sand dome. The air horns won't last more than a few months, and the whistle would be moved forward, adjacent to the horns' former position. I count nine people in this photo, possibly a crowded cab on this date.

The ( 1927 ) M-4's quickly proved their superior abilities, and in 1929 six additional locomotives of the class were purchased - engines 6322-6327. They were identical to the first group except for a few small details. They had more modern sloping-front cabs, while the original group had vertical-front rectangular cabs, and the new engines had traditional steam whistles mounted on the large outside steam pipe on the left side of the boiler, just ahead of the forward sand dome ( however, in this photo, the whistle is just ahead of the rear sand dome ). The air horns of the earlier M-4's were soon replaced with steam whistles too. Finally, all six of the new locomotives were equipped with Worthington BL feedwater heaters.