I could see repurpose from subway equipment, since it’s already powered and usually set up for short trains. America doesn’t have too many above-ground EMU (electric multiple-unit) models to work with; the 50-year-old Metroliner comes to mind, and some Pennsylvania Railroad predecessors.
The infamous British Rail Pacers were effectively bus bodies with flanged eheels, so that sort of retooling has precedent.
Are you thinking battery, third rail, or cantenary? The latter two are much more infrastructure heavy so you might not see them at the outer edges of a network.
Locomotive hauled trains might be another viable option, one small battery-powered engine (see Norfolk Southern 999) and one or two ancient coaches.