In U.S. 3ft gauge practice, outside frame locomotives came into being about 1900 (a very few in the 1890s). By then, the narrow gauge boom was pretty much over, as were the reasons for using narrow gauge. U.S. 3ft gauge was used where and when:
- significant savings could be obtained by using a roadbed about 2/3 as wide (think mountain and canyon sides)
- interchange was not an issue either due to location (no other railroads near) or era when started. Universal interchange in standard gauge didn't really get started until the 1880s and really took off in the 1890s to 1900 as things like couplers and brakes were standardized. Also, labor to unload/load cars became more of a factor, and shipments were going much further (outside the local region) as the rail system expanded. Interchange costs probably did more to kill narrow gauge than anything else.
- the size of narrow gauge rolling stock was greater then 75% of the equivalent standard gauge cars. This was the other economic downfall of narrow gauge. One of the assumptions was that you could get 80-90% of the capacity of standard gauge for 75% of the cost. In the 1870s, there was some truth to that premise. But by 1900, the growth in capacity of standard gauge rolling stock and train length had turned the economics on its head.
In an attempt to grow train length and capacity to something approaching standard gauge again, some of the narrow gauge railways tried using steel underframes for their cars, and buying larger engines. The path provided by Baldwin and others to larger engines was outside frames (and trailing trucks) which enabled larger fire boxes and steaming capacity. The fireboxes on older inside frame locos were limited in size by the space between the rails. Almost all of the U.S. 3ft gauge outside frame locomotives were purchased between 1900 and 1925.
my thoughts