A ~30 minute trip does not require the conductors, attendants or kitchen staff of, say, the two day trip the California Zephyr takes.
The overhead of those workers' wages would scale roughly with ridership. Amtrak ridership is up dramatically this year[0], but it is still 20x smaller than for domestic air travel[1]. They employ nearly 5000 people in mechanical and maintenance roles[2], which is a cost largely independent of ridership.
You can argue the case that Musk's cost estimates are overly optimistic, but Amtrak's operating costs are largely relevant.
The overhead of those workers' wages would scale roughly with ridership. Amtrak ridership is up dramatically this year[0], but it is still 20x smaller than for domestic air travel[1]. They employ nearly 5000 people in mechanical and maintenance roles[2], which is a cost largely independent of ridership.
You can argue the case that Musk's cost estimates are overly optimistic, but Amtrak's operating costs are largely relevant.
[0] http://www.sacbee.com/2013/03/01/5227255/amtrak-ridership-up...
[1] http://www.transtats.bts.gov
[2] http://www.amtrak.com/ccurl/533/3/Amtrak-Mechanical-Services...