Old Bob
Are you looking for pre-WW1 or WW1 and after information? WW1 or 1910 is a dividing line that is commonly used to separate early rail from the standard practices that prevailed after that. Pre-1910 is a fascinating time of development, engineering, and learning - and is obviously my favorite. Ways of life that we take for granted were just not known then. Over 10% of the labor force worked for the railroads or their direct suppliers in 1900. Cities were choking on pollution (dust and waste) from horses. The new-fangled automobile held the promise of cleaning up the cities.
The American or 4-4-0 became a standard about 1860, but was further refined for about the next 50 years. I can't think of any common carrier railroad - standard or narrow gauge - that did not have at least one 4-4-0 during the second half of the 19th century. They proved to be a little too small and light for Western mountain railroading (and Eastern coal hauling), though.
The 2-6-0, 2-8-0, and 4-6-0 all became mainstay upgrades, depending on the biases of a given railroad's engineering department.
In the very early 1900s, trailing wheels to support larger fireboxes were engineered. Thus started the development of 20th century steam power - the 4-4-2, 4-6-2, 4-6-4, 2-8-2, and so on.
Geared locomotives (Shays, Climaxes, Dunkirks, Heislers, etc) were developed in the late 1880s for logging railroads; their use became widespread in places with steep grades, sharp curves, poor track, and heavy loads. Geared loco production peaked just before WW1.
Janney knuckle couplers and Westinghouse air brake systems were invented, and were finally required in interchange by 1903. Grab iron, steps, and ladder placement were also somewhat standardized by the same act.
The use of air brakes and knuckle couplers, combined with the growing size of locomotives, allowed control of much longer trains. Wood frames and truss rods were no longer up to the task; steel underframes were used for almost all new construction after 1910, and were eventually required for interchange use.
Narrow gauge was incredibly popular in the 1870s and 1880s. However, by 1910 most of the profitable narrow gauge routes had been widened to standard gauge. Narrow gauge had become an economic failure due to the growth in efficiency of standard gauge, and the necessity for economic interchange. The rest of the narrow gauge lines that hadn't already failed would do so in the 1930s and/or 50s.
my thoughts, your choices