1904 GWR 4-4-0 No. 3440 City of Truro
City of Truro is perhaps at the centre of the longest running controversy surrounding the performance of any locomotive. That debate originates from May 9 1904, when No. 3440, with Driver Moses Clements at the controls, hauled the mail from a transatlantic steamer over the 128.1 miles from Millbay Crossing, Plymouth, to Pylle Hill Junction, Bristol, in 123 minutes and 19 seconds. The run was timed by Charles Rous-Marten, and on the descent from Whiteball to Wellington he claimed to have recorded a maximum speed of 102.3mph immediately before a sharp brake application was made. This was because it is said there were platelayers on the track ahead who were slow to move to one side.
Timing details are available for this run, but they have long been the subject of endless debate by interested parties. Also subjected to much debate has been the manner in which Rous-Marten recorded the times, and whether he was a reliable and accurate timer. Cecil J. Allen once went on record commenting that Rous-Marten’s figures for this run were “few and contradictory”. He also suggested that a milepost may had been misread. The critical part of this run is the steep climb to the summit at Whiteball, where Rous-Marten claimed the train was travelling at 62mph, followed by an astonishing acceleration down the average 1-in-96 grade, (maximum 1-in-80, easiest 1-in-127), towards Wellington that culminated in the claimed 102.3mph under 3½ miles from the summit, before the brakes were applied, reducing the speed to around 80mph. Over such a relatively short distance, where high speed was certainly reached followed by sharp braking, debates on what maximum speed could have been reached would be endless, and show no sign of stopping! I do not propose to cover those debates here, as I believe that there is a much simpler way of determining the most likely maximum speed achieved by ‘Truro’ - by reference to the required horsepower. City of Truro has a saturated (non-superheated) boiler, with a maximum indicated horsepower similar to other contemporary 4-4-0s: approaching 1,000ihp. That maximum is known to be in the lower speed ranges, (less than 50mph), for this 4-4-0. As a steam locomotive’s speed increases past its peak power output, the maximum indicated horsepower falls. For City of Truro, that’s around 700ihp at 90mph, falling more at higher speeds. There is some debate about the actual weight of the train, with estimates ranging from 118 tons to 148 tons. But even if we assume the lowest weight, the figures don’t corroborate the 100mph claim. With a speed of 62mph at Whiteball, the average indicated horsepower to accelerate a 118-ton train to over 100mph in less than three and a half miles would be around 1,400ihp; quite a lot more than City of Truro’s absolute maximum. If the weight of the train was 148 tons, the required horsepower rises even more above that impossible 1,400ihp level. And it rises yet again if the starting speed of 62mph is discounted, and was the 52mph that Messrs Allen, Nock and others viewed as the maximum, as supported by the timings.
“TO CONTINUE ACCELERATING THROUGH THE 100mph MARK ON THE BASIS CLAIMED BY ROUS-MARTEN WOULD HAVE NEEDED A FINAL IHP OF NEARLY 3,000! ”
To continue accelerating through the 100mph mark on the basis claimed by Rous-Marten would have needed a final ihp of nearly 3,000! Detailed, sensible calculations show that City of Truro with a 118-ton train could have developed sufficient horsepower to have just got into the low 90s, maybe a 92mph maximum. With a 148-ton train, the maximum may not even have reached 90mph. A ‘City’ would need to produce horsepower so far above its maximum that any consideration of higher speeds unrealistic. Determining the maximum likely speed achieved by horsepower calculations uses a factual approach, where the only conclusions I have ever seen is that City of Truro did not reach 100mph. Contrary to that is the ‘emotional approach’, followed by many who look at the claimed maximum speed as what they would like to be true, not what was possible.
DID IT REACH 100MPH?
City of Truro’s likely maximum hauling a 118-ton train was 92mph. 90mph if the train weighed 148 tons. City of Truro is therefore no different from NYC No. 999 - preserved because of an inaccurate speed claim, albeit claims that have at least ensured they were saved from scrap, allowing them to remain as fine reminders of early 20th century steam locomotive technology.