| Milwaukee Road Class A | |||||||||||||||||||||||||||||||||||||||||||
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 A postcard depicts the Milwaukee Road class A #2 in 1935. | |||||||||||||||||||||||||||||||||||||||||||
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TheMilwaukee Road Class "A" was a class of high-speed,streamlined4-4-2 "Atlantic" typesteam locomotives built by theAmerican Locomotive Company (ALCO) from 1935 to 1937 to haul theMilwaukee Road'sHiawatha expresspassenger trains. Numbered from No. 1 to No. 4, they were among the last Atlantic type locomotives built in the United States as well as the largest and most powerful. The class were the first locomotives built for regular operation over 100 mph (160 km/h), and the first class built completely streamlined, which they kept their entire lives. Although partially supplanted by the largerclass F7Hudsons from 1937, they remained in premiere service until the end. Locomotive No. 3 was withdrawn in 1949 and cannibalized for spare parts to keep the other three running until 1951.
Designed for a6+1⁄2 hour schedule betweenChicago and St. Paul, the class proved capable of handling nine cars on a6+1⁄4 hour schedule. The only change was the addition of aMars 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 was designed to cruise at over 100 mph and be able to achieve 120 mph (190 km/h). A run with adynamometer car behind the locomotive was made on May 15, 1935, by locomotive No. 2 betweenMilwaukee andNew 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 theLNER Class A4 and GermanBR 05 for the title of fastest steam locomotive until that time, but no official records have been recovered. The successorMilwaukee 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) diameterdriving 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 thesmokebox.
| Milwaukee Road no. | Alco serial no. | Built | Retired |
|---|---|---|---|
| 1 | 68684 | May 1935 | November 1951 |
| 2 | 68685 | May 1935 | November 1951 |
| 3 | 68729 | May 1936 | September 1949 |
| 4 | 68828 | April 1937 | June 1951 |
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