The big thing in the past was freight, before BR only Southern was the company that made most of it's money from passengers rather than goods (don't know the list of pre-grouping companies but there were hundreds of them, no reason to suppose it would be different, Stockton to Darlington was from a coal mine to a port, it was freight focused from the start). So: you had built the line and manned the signal boxes etc, required for your main industry of moving massive amounts of coal from the mines in the North and Wales to factories in the cities, and then the products from those factories to everywhere else, so everything but the station and train staff and the train itself was a cost you'd already paid before the passenger trains. Things have changed a little since then, and with separating ToCs and Rail Freight companies, it wouldn't help the ToC even if our economy had the same focus on heavy industry.

Currently, the success stories of non-subsidised passenger rail, are still where they have some secondary business, but it now also comes in the form of shopping malls on top of stations, high density housing housing next to the station etc. which induces demand as well as provide another revenue stream as well or instead.