Ridiculous compared to what? Freeway expansion? Airport expansion? Do nothing?
Freeway expansion is not that much cheaper, and if you factor in the cost of 6 hour drives (or 8 hour bus) over the entire users, I’m not sure freeway expansion comes out in favor.
Airport expansion is also ridiculously expensive. The airspace between San Fransisco and Los Angeles is already super congested. You will probably need to build whole new airports to offer the same capacity as high speed rail. Airport expansion also fails to service the Central Valley, which leads to further economic depression of the millions of people who live there, making this option even more economically ridiculous.
This leave us with do nothing. Sure people can take the 9 hour bus or the 12 hour train and save the carbon footprint, or they can ignore the climate crisis and drive the 6 hours or navigate the dozens of airport combinations. This is by far the cheapest option, but only if you ignore the economic impacts of people choosing not to travel between between population centers in California. Given the cost of travel in California, both in time and carbon emissions, than keeping the travel options as is, is also a ridiculous option.
Perhaps high speed rail is economically ridiculous, but given the options we have, it is still the most sane option.
