Fastest steam locomotive

The highest speed ever ratified for a steam locomotive is 125 mph (201.16 km/h), with a brief spell at which the speed reached 126 mph (202.77 km/h), by the London North Eastern Railway 'Class A4' No. 4468 Mallard, which hauled seven coaches weighing 243 tonnes (535,722 lb) down Stoke Bank, near Essendine, between Grantham, Lincolnshire, and Peterborough, Cambridgeshire, UK, on 3 July 1938.

The speed was timed over 402 m (440 yd). The locomotive's configuration was 4–6–2, and it was later numbered 60022.

There is some controversy surrounding the claimed speed of 126 mph. The speed of Mallard's record-setting run was measured using a device called a dynamometer, which recorded various parameters as marks on a scrolling sheet of paper that was fed through at rate of 24 inches for every mile travelled. For purposes of the record, the most important measurements are the ticks representing time (one every half-second) and the ticks representing distance covered. The data on the latter line included ticks that were added automatically every quarter-mile as well as ticks that were entered by two technicians, who pressed a button whenever they passed a milepost or other trackside landmark. The record-breaking speeds were attained between mileposts 93 and 90.

The figure for the speed that Mallard attained was calculated by measuring off the distances between these various ticks. At the time of the attempt, engineers examining the roll of paper gave Mallard's top speed as 125 mph (a speed that the train maintained for around five seconds just before it reached milepost 90. A later re-examination, however, led to the top speed reached being revised up to 126 mph, although the train was thought to have only touched this speed for around a second (or for a distance of about 180 feet). Mallard's designer, LNER Chief Engineer Sir Nigel Gresley, preferred to stick to the original 125-mph measurement.

Subsequent re-examinations of the paper rolls, most notably by railway historian David Andrews, have suggested that Mallard's maximum speed should perhaps be revised down to 124 mph (199.55 km/h). He found slight errors in the original measurements, as well as evidence of an almost imperceptible variation in the speed of the paper feed mechanism used by the dynamometer. Adjusting for these irregularities produces an smooth acceleration curve without the unlikely bursts of speed suggested by earlier analyses.

There are a few potential rivals to Mallard's record, though the only other one whose speed was measured with precision equipment was the Deutsche Reichsbahn-Gesellschaft No.05 002 locomotive, which recorded a speed of 200.4 km/h (124.52 mph) while pulling a short train between Hamburg and Berlin on 11 May 1936. It should be noted, however, that the No.05 002's speed was recorded using a dynamometer running at a feed rate of just 150 mm per kilometre (equal to 9.5 inches per mile), increasing the potential error in the measurements. Furthermore, the paper dynamometer roll from that run has been lost, so a modern re-examination is not possible.

It is possible that Mallard's 83-year-old record will eventually be broken, however. A team in the United States are constructing a new Pennsylvania Railroad T1-class locomotive, which they're calling PRR T1 5550. The T1-class was the last steam locomotive made by the Baldwin Locomotive Works, going into service during the twilight years of steam in the late 1940s. Although anecdotal reports suggest that these powerful machines were capable of speeds of up to 140 mph, no T1-class ever made an authenticated record attempt. The team hopes to claim the record when the 5550 is complete.