I think that is from The Adventure Of Silver Blaze, and refers to a journey from London to Devon, which even in those days was a backwater. I seem to recall that in The Three Garridebs it is put that a train-journey from London (Euston) to Birmingham - about 112 miles - took slightly less than two hours, which is about the same time as it takes these days.
Someone said that a Bradshaw's Guide is a rare book these days - so glad to hear that, as I've got one dating from about 1865! I'm rich - rich! Do you hear me? Rich! Hah, hah hah!
We are suffering from a plethora of surmise, conjecture and hypothesis. - Sherlock Holmes, The Adventure Of Silver Blaze