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.