He gets how important the Liverpool and Manchester Railway (1830) was. I've pointed to that for years as the moment when the Industrial Revolution got out of beta. There were railroads and steam locomotives before that, but they were one-off demos or in-house operations. The Liverpool and Manchester was all steam locomotives (no horse-drawn vehicles), with double tracks, signals, switches, schedules, and tickets. At last, people could buy a ticket and go someplace. As the author points out, once it got running, it was quite profitable.
So, of course, everybody wanted to copy it. Hence the railway mania. I'm amazed that the author had trouble finding materials on that subject, but apparently he searched for the phrase "railway mania" and didn't get enough hits. Strange. Oh, and on page 189, there's "(in fact, got continued growth)", a phrase from an outline never filled in.
Brunel was indeed a high-cost builder. Most of his works are still standing and in regular use, even those coming up on two centuries old. Railway mania was enormously profitable for Britain, although many railroad stockholders did not do as well. Railroads probably paid off better than aviation - commercial aviation, over its entire life, has been a net loss to investors.
If you want to read about 19th century railways, both Stanford University and the San Francisco Mechanics Institute have good collections.