Stadler didn't bid an off-shelf FLIRT import for the T's EMU RFP. It bid the Caltrain FrankenKISS only, to help recoup some of the overcustomization losses for that West Coast boondoggle. The industry doesn't trust U.S. transit agencies to manage procurements responsibly, and responds with a bear market on the actual bids. That bear market and the inherent mistrust the industry has of the U.S. has been covered extensively by the transpo blogosphere, and it's not necessarily the T's fault because their debut EMU RFP is so colored by bad experiences builders have had with other irresponsible agencies.
Ultimately, the Rail Vision can only work with specs on the makes they were bid. They were bid slim pickings.
EDIT: The FLIRT has no makes in its entire product family that board at 48-inch full-high platforms. It's entirely a low- (8-inch) and mid- (up to 29-inch) height platform make. So why would TM ever want to base its performance specs on a vehicle that is totally inapplicable to anywhere on any NEC member's system???
EDIT 2: It's the intercity variant of the FLIRT that has the brawniest acceleration profile in Stadler's product catalog. The commuter variants accelerate about half as fast, not much different from the commuter makes that the T was bid in its RFP (like the Silverliner V, which can reach 100 MPH in the absolute but only get to that point relatively slowly on-the-clock). So again...premium-class $$$ on the procurement to net TM's desired performance profile, something the T cannot afford. Can't afford even if the manufacturer were to come up with a variant (which they have so far declined to bid) that can square the U.S. East Coast's platform height. With a cost of considerable rider discomfort when commuter seating/livery and standee areas are applied to the maximum G-forces exerted by intercity-class propulsion. This is a basic, basic flaw in all of TM's "Extreme Train Sim" traffic modeling across the system.
