To be a bit pedantic, but completely accurate, a 'loss leader' is a product a company is prepared to lose money on in order to lead customers to more profitable products and thus increase profits (eg cheap bread/milk brings in the punters who they hope will buy other groceries). A bus route that never will make money is a complete loss to profits.
Ah - now I have come across sections of a route which don't make money, but which continue to run because they bring in a handful of people who stay on the bus and are joined by others which makes the bus route profitable as a whole. The classic is/was the route from
Bath Bigcity to
Easterton Tinyvillage via
Melksham Biggertown,
Devizes Bigtown
Potterne Village
West Lavington Village and
Market Lavington Village.
The metric here are that you will never pay your way from Easterton to Market Lavington, but that helps fill the bus the rest of the way, and no matter how far you trip that route back towards Bath you're going to damage your income for the inner sections. The recent history of the route is a complex one, with First running the service in the days I actually lived in Easterton; they cut it back all the way to Melksham and FareSaver took over the longer route by extending their Bath to Melksham bus, initially with a change and a smaller vehicle onwards from Devizes - commercially run but perhaps a loss leader in order to get a good load of transfers. People don't like changing, though, and the outer section is now a through / larger vehicle ... which I doubt is ever going to make a profit on the very final section to Easterton ...