For the sake of this exercise, I'm taking a few liberties. First, that the Bay Area had valued its rail transportation system from the 30s to the present, but had consolidated it all, as well as the Golden Gate Bridge, under the single umbrella of the MTC. Second, that European best practices had been implemented at least in this corner of the country. Third, that the Interurban could now survive on a 50% subsidy. And fourth, that Marin and Sonoma have their current populations, though with less sprawl.
Though I had originally intended for this to be a bit more a light post rather than something more data-driven, a Twitter conversation with Dan Lyke motivated me to put some numbers behind the costs of an Interurban.
Costs per vehicle-kilometer (vkm) vary widely based on the system. Vancouver's automated Skytrain system costs $2.18/vkm, BART costs around $3.50/vkm, and New York's subway costs $5.81/vkm. Using quite a few assumptions, I get an average annual operating cost between $43.2 million and $111.6 million. If we assume an average fare of $2.50 and a 50% farebox recovery rate, total ridership would need to be between 8.6 million per year, roughly the same number of transit trips on today's GGT system, and 22.3 million. With the Geary and North Beach extensions (Muni's 38-Geary alone carries over 13 million weekday passengers per year), it's entirely feasible for the system to meet BART's 80% farebox recovery.
Alas, reconstructing the system would be prohibitively expensive and politically impossible. Large portions of some major roads (Sir Francis Drake Boulevard, Fourth and Third Streets, Magnolia Avenue, Miller Avenue, and others) would need to be converted back to rail, wealthy homeowners would need to accept trains running behind their back yards, Sausalito would need to take a new elevated railway along the waterfront, Geary and North Beach would need to be torn up for a new subway, and over $10 billion would need to be spent. While the San Francisco part of the project might be worth it, for 8 million riders per year, most of them already served by transit, the cost and pain of the Marin Interurban simply wouldn't be worth it.
This map, along with all my other maps, is posted in the Map Room.