Firstly, who actually questioned the bit about tolerance?  Do they understand what Engineering tolerance is about? (that is why it was WIkilinked).  The source itself used the wording "for example ...".  Does anybody who has a modicum of understandign of engineering really think that that the tolerance in the UK is going to be less than  of what it is in the US?  Maybe it is best to remove that citation altogether. 
I don't understand your comment about the standard gauge - here is a cut and paste from the source document (irrelevant material removed):
2.2.1.1 ...
a) ...
f) Track gauge variation from standard (1435 mm) gauge.
I disagree with you about WP:PRIMARY - I refer you to my earlier quote on this page. No expert advice is needed to understand the relevant part of the 1846 Act.