Milwaukee Road class A

The Milwaukee Road Class "A" was a class of high-speed,

Designed for a 6+1⁄2 hour schedule between Chicago and St. Paul, the class proved capable of handling nine cars on a 6+1⁄4 hour schedule. The only change during their service life 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 100 mph (160 km/h) or greater was required to keep these schedules; the class A locomotives were designed to cruise at over 100 mph and be able to achieve 120 mph (190 km/h). 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 14-mile (23 km) stretch the speed of 112.5 mph (181.1 km/h) 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 110 mph (180 km/h). There are reports that these locomotives could exceed 120 miles per hour (190 km/h). 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 125 mph (201 km/h).

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 300 psi (2.07 MPa), 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 84-inch (2.134 m) 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

• Benn, Bryan. "Fastest Steam Locomotive". Archived from the original on 2006-04-20. Retrieved 2006-01-09.
• Ross, Don. "Chicago Milwaukee St. Paul & Pacific: Class A 4-4-2 Hiawatha Atlantics". Don's Rail Photos. Retrieved 2006-01-09.
• Hollingsworth, Brian & Cook, Arthur (2001). The Great Book of Trains. Osceola, Wisconsin: MBI Publishing.
  ISBN 0-7603-1193-5
  .
• Reed, Brian (1972). Loco Profile 26 - The Hiawathas. Windsor, Berkshire, UK: Profile Publications Ltd.