Just to throw fuel onto the fire here is a quote from a thread on another site, I cannot verify its accuracy-
''The 126 mph figure corresponds to a blip on the speed graph, with physically impossible accelerations. It must have been due to a jolt, or,
according to British railfans who unearthed a second speedgraph copy without the bump, later doctoring.
Indeed the railway engineers themselves (including designer Sir Nigel Gresley) certified 125 mph (201.2 km/h) as the true top speed achieved - but that is within the margin of error with the other two contenders below.
The Mallard achieved the record downhill, and
with modifications unfit for more than a single run - indeed the Mallard broke down at the end of the record run with a hot axle.''
There is another issue here- is it a world record or a world fastest time. As I understand it you cannot have a world record in the Athletics Marathon because of the huge variation in conditions- just a world fastest time. By contrast the world land speed record is measured using a set of pre-determined criteria to try to achieve some level of uniformity.
What is undoubted is that Mallard is [almost] universally quoted as having achieved the fastest speed by a steam locomotive. There is little doubt that other locomotives had the ''potential'' to go faster, and may have done so without any recording equipement on board.
The other two contenders referred to in the thread were-
Deutsche Reichsbahn's [class] 05 [no.] 002
Milwaukee's "Hiawathas".
Here is the website-
http://www.eurotrib.com/story/2005/10/10/4437/7476