Designed for a hour schedule between
Chicago and St. Paul, the class proved capable of handling nine cars on a hour schedule. The only change was the addition of a
Mars Light beneath the winged emblem on the nose in 1947. They hauled the fastest scheduled steam-powered trains in the world. Running at or greater was required to keep these schedules; the class was designed to cruise at over 100 mph and be able to achieve . A run with a
dynamometer car behind the locomotive was made on May 15, 1935, by locomotive No. 2 between
Milwaukee and
New Lisbon, Wisconsin. Over a stretch the speed of was recorded. This was the fastest authenticated speed reached by a steam locomotive at the time, making No. 2 the record holder for steam traction speed and the first steam locomotive to top . There are reports that these locomotives could exceed . Such speeds would have put the class A in contention with the
LNER Class A4 and German
BR 05 for the title of fastest steam locomotive until that time, but no official records have been recovered. The successor
Milwaukee Road class F7 was even more powerful, with a claimed top speed of . The design was fairly conventional but unusual in some aspects. One goal was reducing reciprocating mass, which could not be completely balanced. This was the reason for the high boiler pressure of , which allowed smaller pistons. The reciprocating mass of the connecting rods was also reduced with the use of four driving wheels rather than six. The main rods connected to the first pair of driven wheels rather than the (more conventional) second; again, this reduced the reciprocating mass as well as providing more even power throughout the stroke. The large diameter
driving wheels reduced piston speed and made high speed less taxing on the machinery. The streamlined casings were designed to open easily for servicing; the front end had clamshell doors ahead of the
smokebox. ==References==