The success of the @MBTA's bus electrification strategy depends on 3 things: wanting to do it in a reasonable timeframe, engaging in good faith with advocates, and looking at world best practice.
Not to mention there are many short routes and express routes where the limited capacity is not a concern.
Then there is in-route charging where buses can quick charge during a layover. https://www.utilitydive.com/news/high-speed-hurdle-designing-a-fast-charging-network-for-electric-buses/553054/