That's a funny comment. The colonies had a smattering of railways, most colonial subjects probably never saw any railways or trains. Heck, for an example, look at how the African railway networks look in 2019 (!): https://en.wikipedia.org/wiki/African_Union_of_Railways#/med...
This is an odd tangent, though, if you are interested in humanity's ability to do fine metalworking etc. and how this influenced what transport technologies were viable in what year. This isn't about rural backwaters, anywhere. It's reasonable to define mass adoption to mean that millions are using the thing, although obviously the first million will be in advanced bits of the world.
The keyword being "UK". We can include the US and a bit of Western Europe plus Japan.
For much of the world a train was exotic even far into the 20th century.