:::: Well, there is a huge amount of good material in this article, but there are some elements that detract from that, and may deter the casual reader. Here are some areas that I would wish to improve: 

 :::: I was a bit surprised to find Shipway and Paxton were not among the referenced sources. In fact the first two references give "Oops, page not found" which does nothing for the authority of this article.  

 :::: The article spends a lot of time discussing other bridges, and (as has already been pointed out) discussing the K&TR; in general. The K&TR; article itself is very weak, and this article is actually a better history of the K&TR;, but that's not the point. We need to be clear (in the first few lines) whether we are describing the original Viaduct, or all three. (I believe Network Rail consider the one that carries trains today to be "Laigh Milton Viaduct".) The lengthy "Evidence from OS maps" section sounds defensive, as if we aren't really sure we have got the right location. "The new bridge required embankments on either side ..." er ... which bridge is this? We know, but the casual, uninformed reader doesn't. 

 :::: Some of the text detracts from the subject matter: Consider "The Portland Bridge at Kings Mill Viaduct of 1817–19 is believed to be the oldest railway viaduct (as distinct from a single-span bridge) in England, and the fourth oldest railway bridge of any kind in the United Kingdom. The three known older examples are Causey Arch in County Durham (1727), Ticknall Tramway Bridge, Derbyshire (1802) and Laigh Milton Viaduct (1811). The engineer of the second and third of these was William Jessop." Crikey. 

 :::: There are a couple of "citation needed" and "disambiguation needed" markers in the text, and some red links; these do nothing to persuade a reader that this is an authoritative article. 

 :::: The word "spandrels" ought not to be in quote marks  it's a perfectly respectable term. The stone blocks aren't "sleepers" ... 999 people out of 1000 would understand "sleeper" to mean a beam that carries both rails; they are "sleeper blocks".  And the C&HPR; rails aren't "plateway track", they are edge rails. 

 :::: Let me repeat, the core of this article is excellent, and better researched than a lot of railway-related stuff on Wikipedia. But the above refer to the areas I would like to improve.