Aha, MEC has provided the keywords I was looking for: "NGL Supply Terminal Co.".
Using that company name, some searching brings up an article from September 2015 that fills in some of the gaps I had (excerpts):
As for NGL, searching more turned up a listing of Active Fuel terminals as of June 2020 (recent enough), and while NGL has several facilities listed, they are in Indiana, Minnesota, Arkansas...but not Maine. There are several terminals listed for Maine, perhaps the propane cars which used to go to the Waterfront are now going to one of them.
Using that company name, some searching brings up an article from September 2015 that fills in some of the gaps I had (excerpts):
NGL Supply Terminal Co. is moving forward with its plan to move from Commercial Street in Portland to Rigby Rail Yard.The article included a schematic showing the proposed layout of the terminal, overlaying the turntable pit.
...
[NGL]operates just west of the Casco Bay Bridge in Portland. The lease on that property expires this year [2015]
...
Fitzgerald said. “Our relocation is motivated by and in direct support of the state’s redevelopment of the Portland waterfront and we’ve worked closely with the federal, state and local officials to carefully select the Rigby Yard location.” (oops, as MEC pointed out, I guess not close enough)
...
The storage tank would be built just below what previously operated as the rail yard turntable
As for NGL, searching more turned up a listing of Active Fuel terminals as of June 2020 (recent enough), and while NGL has several facilities listed, they are in Indiana, Minnesota, Arkansas...but not Maine. There are several terminals listed for Maine, perhaps the propane cars which used to go to the Waterfront are now going to one of them.