I think smaller trains with frequent service is the model that would fit best for SMART. Comparing ACE to SMART isn't feasible since ACE does not run an all-day schedule with frequent service in both directions. Using a loco plus one or two bi-levels wouldn't be very effective compared to a DMU.

About "zero-emission" locomotives: "there is a prototype" does not mean "it's ready for dependable service". Certainly interesting but why be the guinea pig?

Dunno if SMART have considered them, but modern railcars with ability for passengers to pass through the length of a train consisting of multiple units certainly do exist. The look takes a little getting used to, but "form follows function" can be beautiful too. It uses standard components from the bus and truck industry for transmission and engine parts, so many spare parts are easier and cheaper to procure.