It's a big energy waster if the door separating the passenger area from the vestibule is always open, and has lost its ability to self-shut from the centrifugal forces of the train moving. The single-levels with their double sliding doors are especially bad for that. Don't see it as much with the bi-levels and their single sliding doors, unless some passenger is actively propping it open (which the staff discourages when they see it). The bi-levels tend to fare better when there's a temperature extreme outside because they chose a better sliding door mechanism design, and with all of them being newer and/or way more recently overhauled than the flats maint thoroughness also plays a selection role. There are way fewer 'loose' sliding doors on the bi's because the mechanisms are uniformly newer, whereas they simply don't give a crap what minor fittings are worn out this week on the flats. To the extent it's taxing their electricity usage, that's a thing stepping up the sliding door maint practices would probably cure quite effectively enough. But good luck finding an enforcement mechanism in Keolis' contract to entice them to go that extra mile. ¯\_(ツ)_/¯
Vestibules aren't supposed to be perfectly climate-controlled to begin with. Conductors delay the door closings for safety lookout points when the train is in motion, for instance. And those closing delays are usually longer when the doors are being manually instead of automatically closed, so purging the last of the manual-door cars (Bombardier flats, mostly) and getting more all- level-boarding lines on the schedule would help there. More fully-insulated articulation starts getting expensive when commuter trainsets are being pulled apart and mashed together for length-to-schedule matching on a daily basis. The rate of splitting/combining would wear out those fortified plugs a bit quick. Amtrak has much more overbuilt car-to-car portals because of the higher service class and the climate extremes that any given car in a continent-wide fleet could be subjected to, and much more precise attention to HEP electricity budgeting given how much their HVAC gets taxed by single schedules passing through multiple climate zones. Simplest explanation for that: it pays off for Amtrak and Amtrak-usage HEP electricity budgets to invest extra for the vestibule climate regulation, doesn't pay off for most Commuter Rail operators in the country who end up buying very nearly the same handful of generic coach livery configurations across their service class.
We can do a bit better culling the systemwide cruft (old cars, slacky maintenance, un-upgraded platforms that force manual-door usage), but vestibule redesigns are probably not going to help nearly enough to be worth the cost/maint premium.
