Editing LNER Class A4 (section)

==World record==
[[File:Mallard Record Plate 01.jpg|thumb|left|Plaque on ''Mallard'' in commemoration of breaking the previous world speed record of 124.5 mph]]
On 3 July 1938, [[LNER Class A4 4468 Mallard|4468 ''Mallard'']] - the first of the class to enter service with the [[Kylchap]] exhaust - pulling six coaches and a [[dynamometer car]], set a world speed record (indicated by the [[dynamometer]]) of {{convert|126|mph|km/h|1|abbr=on}}. Gresley never accepted it as the record-breaking maximum. He claimed this speed could only have been attained over a few yards, though he was comfortable that the German speed record of {{convert|124.5|mph |km/h|1|abbr=on}} had been surpassed.<ref>{{cite book|last=Hughes|first=Geoffrey|title=Sir Nigel Gresley: The Engineer and his Family|year=2001|publisher=The Oakwood Press|isbn=0-85361-579-9|page=147}}</ref> Close analysis of the dynamometer roll (currently at the NRM) of the record run confirms that ''Mallard'''s speed did in fact exceed that of the German [[DRG Class 05|BR&nbsp;05&nbsp;002]].<ref name=Speed>{{cite book |title=Speed on the East Coast Mainline |first=P |last=Semmens}}</ref>{{rp|64}} The ''Mallard'' record reached its maximum speed on a downhill run and failed technically in due course, whereas 05&nbsp;002's journey was on level grade and the engine did not yet seem to be at its limit.<ref name="FastestSteamLoco">{{Cite web |url=http://www.germansteam.co.uk/FastestLoco/fastestloco.html |archiveurl=https://web.archive.org/web/20060420181235/http://www.germansteam.co.uk/FastestLoco/fastestloco.html |url-status=dead |title=Fastest Steam Locomotive |archivedate=20 April 2006 |website=www.germansteam.co.uk}}</ref> On the other hand, the German train was four coaches long (197 tons), but Mallard's train had seven coaches (240 tons).<ref name="FastestSteamLoco" />
One fact, often ignored when considering rival claims, is that Gresley and the LNER had just one serious attempt at the record, which was far from a perfect run with a 15&nbsp;mph permanent way check just north of Grantham; despite this a record was set.<ref name=Yeadon73>{{cite book |last1=Boddy |first1=M.G. |last2=Neve |first2=E. |last3=Yeadon |first3=W.B. |editor-last=Fry |editor-first=E.V. |title=Locomotives of the L.N.E.R., part 2A: Tender Engines – Classes A1 to A10 |date=April 1973 |publisher=[[Railway Correspondence and Travel Society|RCTS]] |location=Kenilworth |isbn=0-901115-25-8 }}</ref>{{rp|125}} Gresley planned another attempt in September 1939, but was prevented by the outbreak of World War II.{{r|Yeadon73|p=126}} Prior to the record run on 3 July 1938, it was calculated that {{convert|130|mi/h|km/h|abbr=on}} was possible; Driver Duddington and LNER Inspector Sid Jenkins both said they might well have achieved this figure had they not had to slow for the Essendine junctions.{{r|Yeadon73|p=125}}

At the end of ''Mallard'''s record run, the middle big end (part of the motion for the inside cylinder) ran hot (indicated by the bursting of a heat-sensitive "stink bomb" placed in the bearing for warning purposes), the bearing metal had melted and the locomotive had to stop at [[Peterborough]] rather than continue to London.{{r|Yeadon73|pp=103,126}} Deficiencies in the alignment of the [[Gresley conjugated valve gear|Gresley-Holcroft derived motion]] meant that the inside cylinder of the A4 did more work at high speed than the two outside cylinders – on at least one occasion this led to the middle big end wearing to such an extent that the increased piston travel knocked the ends off the middle cylinder<ref name="cjallen">{{cite book |last=Allen |first=CJ |title=Two Million Miles of Train Travel |isbn=0-7110-0298-3}}</ref> – and this overloading was mostly responsible for the failure.{{Citation needed|date=October 2008}}