One of Musk's original points was that cost of tunneling isn't linear with diameter--double the size more than quadruples the price, or something like that. But then the insistence on using electric cars seems... counter productive.

The London and Budapest subways use very narrow tunnels, about the size of Musk's tunnels. Their engineering is considered anachronistic, but given the cost formulas for tunneling one wonders if planners are being short-sighted or stubborn by insisting on larger tunnels. A subway with narrow cars is better than no subway at all. But then there's probably considerations, like handicapped accommodations (i.e. large wheel chairs) that make London or Budapest-sized subway cars a non-starter today, at least in the U.S., notwithstanding that it'd likely still be far cheaper to build a narrower subway and then pay for private, on-demand road transport for those physically unable to use the subway.