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.

In 1922, in order to meet the urgent need for additional passenger power, the Burlington received eight locomotives of the 4-8-2 wheel arrangement from the Lima Locomotive Works. The engines were numbered 7000 - 7007 and classed as B-1. They were designed to handle the heaviest trains on the line without doubleheading. To meet this requirement, the B-1 engines were equipped with automatic stokers of the Duplex type. Their total engine weight of 350,000 lb., with 235,500lb. of this carried on the drivers, gave them a tractive force of 52,750 lb. The Lima-built Mountain types were designed to burn lignite ( some were later altered to burn bituminous coal ). Although the engines were delivered without feedwater heaters, they were later equipped with Worthington type systems. The weight of the firebox, which was of the radially stayed design, was supported by a Rushton type trailing truck. Four-wheel trucks of the pedestal design were used on the tenders.