Burlington's first diesel locomotive was not, as many seem to believe, the famous Pioneer Zephyr, but rather a lowly switcher from virtually an unknown builder. The Pioneer Zephyr was completed in April 1934, but did not enter regular service on the Q until Novermber 1934. By that time, CB&Q 9121, a diesel electric center-cab switcher, had been in service in and around Omaha for a full year. The 9121 was the first of only three railroad locomotives ever built by the Midwest Locomotive Works of Hamilton, Ohio. All three were ordered by the Q at the same time and they were delivered in October 1933 ( 9121, c/n 1073 ), August 1934 ( 9122, c/n 1074 ) and October 1934 ( 9123, c/n 1075 ). The 9121-9123, weighing 65 tons each, featured a pair of Cummins Model L-6 six-cylinder, four cycle diesel engines rated at 225 horsepower at 1,000 rpm, for a total locomotive horsepower of 450. Electrical equipment was from GE, including a pair of GT-1160-B-1 self-exciting main generators and four 714-B-1 traction motors. As with all the center-cabs, there were operating controls on both sides of the cab, making the locomotive totally bi-directional.

The 9121 was requisitioned by the U. S. Army during WWII, eventually becoming U. S. Army Transportation Corps 7196, later sold to Stockton Terminal & Eastern, becoming that California shortline's No. 10. Both the remaining Midwest locomotives led uneventful careers on the Q, working primarily in the Lincoln-Omaha-Saint Joseph area, and in Illinois. The 9122, after being stored for over a year, was retired on the last day of 1962, and dismantled the following May 17, while the 9123, last used as the Peoria switcher, was sold for scrap to Precision Engineering Co. in November 1965.