TPR37777-
Dunno if your search found anyone, but good going on still being around to post.
Environmental -- particularly wetlands -- protections are relatively recent concerns. Back then and still now, navigation trumped railroads, marine access to places in and around the Lechmere area continuing into the 20th century. Bringing "made" land up to navigation channels required sinking pilings, erecting waling, and back-filling with whatever material could be gotten. The Millers was more a tidal estuary than a navigation resource, so it could be filled more readily. The area could still be mucky enough that pilings were put in as stabilizers. One more inland soil source would have been Asylum Hill, the pre-Belmont site of what would become McLean Hospital. Asylum Hill's real removal came when the Yard 8 and Yard 9 humps just east of the Washington Street UGB were put in before the Depression.
Particularly after WWII, less-than-benign neglect characterized maintenance of channel edging as commercial marine service onto the Charles diminished and then disappeared.
"A gray crossover is definitely not company transportation."