Fairmount Line Discussion

It's apparently a leasing deal, with associated maintenance through the contract. Ultimately, they will need to do as you say, acquiring their own rolling stock and developing appropriate maintenance facilities. But that's for later, once the concept is proven (or not) and they are ready to expand to other lines.

Feeling cautious, given LIRR's test of a battery powered M-7 did not work out as planned.

For $54M, how many years use do they get? How many trainsets? Who makes these not-in-production FRA compliant BEMUs? Where are they going to put charging stations at South Station? Extra labor to attach/detach charging cables? Or is it overhead centenary, and if so why don't they just string wires?

The Fairmount Line is around 9-10 miles in length, with 1 mile already under 25kV just south of South Station.
Both Siemens (fully FRA compliant) and Stadler (alternate FRA compliant) can build BEMUs that can travel 16-18 miles on battery power. Charge the batteries approaching, idling, at departing South Station should charge the battery to 80% in around 15 minutes, and 100% in 45 minutes. 25kV packs a whole lot more power than 240V, over 100 times more at least.
Amtrak is already buying Siemens BEMUs for Empire services, and several European transit operators have bought Stadler BEMUs. Check out Merseyrail brand new Class 777 trains as an example of Stadler BEMUs Metro model. The technology certainly exists. And by the way, Merta is buying Stadler BEMUs with a FLIRT model train.

Here's a youtube video published by Stadler after winning the Merta order for 8 two car trains.
https://www.youtube.com/watch?v=JrtmVS5T4z8
Note, each trainset can be configured as a two, three, or four car set.
Stadler reports a 100% charged battery can go 46 to 65 miles, and a charging time of 25 to 30 minute from 20% to 80% charge.
Additionally, the old Illinois Central catenary at Millennium Station has 1500Vdc vs the 25kV at South Station.
What voltage Merta will use at LaSalle Street Station power catenary isn't known yet.

Caltrain is buying a Stadler BEMU that can also operate under catenary.

"25kV packs a whole lot more power than 240V, over 100 times more at least. "

That depends on the current draw. Power equals voltage times current, so a device drawing 100 amps of current at 100 volts is using the same amount of power as another device drawing 1 amp at 10,000 volts.