This is a volume that's generated mixed reviews: in part I can share the ambivalence, and we'll come to that shortly, but in part the negative reviews criticise it for not being something that it never set out to be. First of all: this is an encyclopedia, not a connected narrative: it's designed to be used as a work of reference, or dipped into, not read coherently from end to end. Comparing it to a narrative history like Wolmar's "Fire and Steam", therefore, rather misses the point. Secondly, this is a work of Railway history, not of railway locomotive history. By that same token, it is even more definitely not a work specifically of steam locomotive history. Hardware is discussed, but not in exhaustive detail, allowing the book to give space to a thousand and one other topics.
The topics that are discussed are immensely varied - shunting procedures, freight traffic, the development of the Railway Clearing House that governed issues such as through running between companies' lines, and music being four that come to mind at random. A particularly strong strand, however, is the development of new intellectual procedures to manage the railway as it grew to dominance. In some cases these intellectual procedures were ways of running the railway from day to day, such as shunting or signalling; in other cases, they were the development of financial and legal instruments that streamlined the construction and financing of these new enterprises. All these are described in detail. Companies' histories are given succinctly and clearly, and for each major city there is a description of how different companies came to serve it and shape the network as it exists today, an extremely illuminating feature set out clearly with the aid of clear maps. Another strong strand is biographical: individuals related to the history of railways in the UK receive a concise biographical outline, typically half a column to a column. The net is cast widely, taking in not merely civil and mechanical engineers but also historians, propagandists and company directors.
As I mentioned above, the slant of the book is not specifically towards hardware, the editors presumably reasoning that there is sufficient specialist literature on this topic already. Keeping the hardware details brief was probably necessary to allow space for all the other information - equally crucial from a historical point of view. Sometimes - rarely, it must be said - this attempt to touch all bases can lead to superficiality: the section on music, for example, would have been better omitted (abandoning the UK focus of the book, it scrapes together various classical pieces such as those by the Strausses, and makes only the merest gesture at covering popular song: failing to mention at all, for instance, Flanders and Swann's threnody on the Beeching cuts, "Slow Train"). At times, too, the attempt to shift the focus away from hardware can lead to emphases that seem slightly perverse. To take one example as emblematic - railway historian George Dow receives only a line or two less space than O.V. Bulleid, chief mechanical engineer of the Southern Railway 1937-1947, well-known for his willingness to think outside the box in design terms and probably a figure of more significance in the eyes of most historians (something I'm sure Dow would have conceded). Sticking with Bulleid, it also seems rather perverse to omit precise details of some of his work: the abortive Leader project, to produce a steam locomotive that rode on bogies with a cab at each end like a diesel, is mentioned by name, but there is only a general mention of Bulleid producing innovative 4-6-2 locomotives for the Southern without naming them (the West Country/Battle of Britain and Merchant Navy classes), despite their being far more successful than the technological dead-end that was the Leader project. At times, too, technological issues require more space than the book is prepared to give them: I went to the section on valve gear in steam locomotives hoping to understand the different types better than before, but one drawing of a few major types is insufficient (what is needed is a diagram of the various phases of the valve action as the wheels rotate) and I could also have done with some information on the perceived merits of different types of valve gear - why did certain types predominate, and when others were tried out what were the perceived defects it was hoped to remedy? (When a certain number of Stanier's "Black Five" engines were built with Caprotti valve gear, something that BR repeated with the analogous Standard 5 class, what was the aim, why were they not all thus fitted, and what feedback came out of the experiment? I'm no wiser.)
This lack of complete detail on technology, I suspect, is what lies behind some of the negative reviews you will read about this. You certainly won't find all railway knowledge in here, and for hardware you will probably need to go to more specialist works. However, when did an encyclopaedia ever claim to sum up all of human knowledge and render supplementary reading unnecessary? In some areas this is bound simply to be a jump-off point, but it gives a good grounding at least in almost all the areas you might want to investigate and provides for hours of happy directionless surfing to boot. Those, I think, are the two chief requirements one has of an encyclopaedia and it satisfies them well.