There's a whole slew of what are effectively network-effect activities (including VC), often with a risk-based element and/or winner-take-all dynamics, in which we have come to expect "tent-pole" successes to bolster numerous indirectly-profitable ventures.
Film, television, publishing, and music are classic examples of this. A huge and often unrecognised role that the gatekeepers play (studios, distribution companies, broadcast and cable networks, presses, recording labels) is to assume risk on the part of individual ventures. VC operates on a similar logic, where of ten efforts, 1 hit and 2-3 merely profitable ones account for another 6-7 failures. (I suspect the actual success rate is much lower, with increasingly decadal megahits seeming prevalent.)
Those are time-based bets.
In geographic networks -- canals, railroads, highways, air travel, telegraph (often co-tenant with railroad rights of way -- 19th century maps of each are fascinating), telephone, electricity, even water, oil, gas, and other pipeline networks -- a few major success/concentration points support numerous others. Transit systems in particular seem to go through an evolution of private venture to public-subsidy, to wholly-publicly-owned.
Though it's more complex than even that.
Postal subsidies played a massive role in supporting rail and early air transport. Through the mid-1960s, the US Post Office ran mail-sorting cars on trains where mail was sorted en route. The introduction of Zone Instant Postal Codes, better known as ZIP Codes, as well as pre-sorted mail, allowed for routing at origin (or regional) postal hubs, rather than in transit. The loss of this subsidy proved fatal to trains.
Many city transit systems -- bus and streetcar -- were cooperative ventures serving factory and office employers, retailers, and real estate developers, more than the actual daily riders of the systems. It was the creation of a viable transport link between locations that induced the resultant value of those locations.
In the Chicago region, a network of electric-powered commuter rail systems emerged in the late 19th and early 20th centuries. Not only the above dynamic came into play, but these transit links created new linear bands of viable "bedroom community" suburbs surrounding the urban core. Transit times from 30 miles out were less than an hour, shorter than today's commutes.
Construction of highways (notably the Eisenhower Expressway) lead to a sudden collapse in 1956.
https://www.american-rails.com/aurora.html
https://en.wikipedia.org/wiki/Chicago_Aurora_and_Elgin_Railr...
Rural electric and telephone service have long been at best marginally profitable, and development of infrastructure (as well as considerable generating and mainline distribution) were enabled through rural electric and telephone cooperatives, supported by surcharges on urban/suburban service.
Private enterprise enjoys cherry-picking profitable areas/regions for operation. Reality is that a functional country and economy require some level of subsidy.
The gyrations of PG&E over the past two decades may be preamble to a transfer to public (state) control of electricity services in California, possibly elsewhere. This may be a more general trend in such services: originating as private operators, shifting to government services over time.