The technical proposal states "using a smaller vehicle size, which allows us to reduce the internal tunnel diameter to less than 40 feet." But in addition to systems that use Bombardier's Innovia stock (JFK Airport, Vancouver Skytrain, Kuala Lumpur), they also mention projects in Singapore, Sydney, and Montreal. Singapore's rolling stock for its automated lines is even wider than our Bredas, while
Sydney and Montreal's are 1/2-inch narrower.
Budgeting for a 15-foot side platform and 3-foot emergency walkway, it would seem to me that 10-foot-wide trains — or at least 9-9.5-foot width — are feasible. Bombardier's Innovia rolling stock seems to be very customizable, with Beijing's Capital Airport Express trains being 10.5 feet wide.