Was thinking about the ‘70s, as one does. In this case changes in rail transit - deregulation, the formation of Amtrak, and the Mass Transportation Acts, which left a situation where essentially all U.S. passenger rail transport was funded by the federal government.

You run across occasional grumbling about this, that proper transportation systems should stand on their own two feet and support themselves but it’s worth pointing out that American passenger rail has never really funded itself at scale through ticket sales and has always been effectively subsidized by other factors, and federal expenditures have been a major part of this from the beginning.

Long distance-city-to-city travel was supported by shipping, obviously - heavy commodity shipments tended to go too slow for passenger transit but mixed-car trains weren’t unusual, and of course all used the same infrastructure. But if that were the end of it you’d expect traveling to take place at whim of the shippers (like stateroom passengers on transoceanic freighters), and not convenient regular daily schedules. In fact, the regularity and speed of inter-city passenger service was largely a condition of - and underwritten by - the renumerative contracts to carry U.S. Mail.

This was no Steam Age innovation - in the Revolutionary Era federal mail contracts were paid handsomely in part to knit the country together by subsidizing shipping and transport between coastal ports. Much as its estranged parent the British Empire similarly linked its colonies using packet ships.

(Another reason these contracts paid so well was to enrich the well-connected figures who were awarded them by the politician friends they helped put in office. Post Office appointments and contracts were the basis of the patronage coalitions that congealed into our political parties.)

Similarly, a lot of early aviation industry and infrastructure was underwritten by the Post Office through the development of Air Mail, a premium service akin to modern UPS or FedEx same-day options. Charles Lindbergh’s transatlantic “Spirit of St. Louis” was named after the home base of his mail route.

(Of course, even with mail carrying many passenger lines were still unprofitable but maintained as a condition of federal regulations and were cross-subsidized by freight - the American federal regulatory state is often associated with the New Deal of the 1930s, but a lot of those “innovations” had plenty of precedent in railroad regulation, dating back to the Interstate Commerce Act of 1887.)

In a similar but non-governmental case of cross-subsidy, lines connecting cities with their surrounding countryside might have been expected to follow traditional agrarian traffic patterns - heavy service at harvest time and market days, little or none otherwise, but travelers enjoyed daily service in large part by piggybacking on shipments of a perishable product - milk - that was sent to the urban center from outlying farms every day.

Commuter lines, connecting workplaces to residential districts using streetcars or what would now be classed “light rail”, might seem distinct in this regard - with no cargo capacity, all they do is transport passengers. In fact, many of these lines were effectively cross-subsidized by real estate development.

Most famously, commuter lines would buy large, cheap plots at the “country” terminus of their line and erect amusement parks. (Coney Island is the most famous remnant of this tradition.) Not only profiting from the parks themselves, the lines were also able to sell tickets to use expensive lines, stations, and rolling stock that would otherwise sit idle on weekends.

But more than that, many commuter lines were built with an eye towards profit not from tickets but from the appreciation of land for commercial development. A developer would buy large tracts of outlying farmland for cheap (by magnate standards), support the construction of a railroad, and once it had been connected to urban centers of employment sell the land more expensively (but less so than closer-in land) for housing.

This had the problem that once the entire tract was sold off there was little angle in maintaining service, or repairs, on the line. (Landlord/developers profiting off rent, more common in Europe, at least had some incentive to keep things going.) This, and not an automotive conspiracy, was a big part of how Los Angeles’ famed streetcar lines collapsed.

(Another part was that as lines lengthened and branched, sharing the same at-grade rails through the same built-up corridors and centered on the same downtown hub, hellacious traffic jams slowed the system to near walking speed. The major selling point for buses - they could take whatever roads they want, rather than all running down the same street that made sense for a single line decades ago - was a real distinction and not just a smokescreen. Big cities like London, NY and Chicago had had this problem years before, which spurred the growth of grade-separated transit - subways and elevated rail. But in Automotive Age LA, what happened was new building spread out away from the jammed clusterfuck arteries, which further undermined the downtown hub-and-residential-spoke system as more and more commuters were traveling between a “not downtown, or terribly close to the streetcar line, really” home and a similarly located workplace.)

Funicular railroads like LA’s Angel’s Flight, which is essentially a glorified escalator, sometimes made their money off passengers. I’ll give you that one.