1904 GWR 4-4-0 No. 3440 City of Truro

Steam Railway (UK) - - News -

THE CLAIM

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.

ANAL­Y­SIS

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.

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