fence on the Boston Common. Behind their cage, yellow dinosaur steamshovels were grunting
Aha. It's a fence our speaker is pressing against. We were gonna say fence. No, really we were.
We get the feeling that what he presses against now—the barbed and galvanized fence (not of the cheery, white-picket variety)—is somehow less appealing than the aquarium glass of his past.
(FYI, the Boston Common is an area in the city's center. Sort of like a plaza.)
Through the fence he watches construction happening. He describes the steamshovels as yellow dinosaurs, trapped behind a cage. This is not completely unrelated to what we've read already in this poem. Remember the "cowed, compliant fish" behind the glass? This description is similar, except the machinery seems more menacing with its "grunting" noises.
Lowell is still on a roll with poetic devices. He's taken a break with the alliteration and has moved on to personification, which is giving inanimate objects (in this case the construction machinery) characteristics of a—you guessed it—person (the "grunting," specifically).
as they cropped up tons of mush and grass to gouge their underworld garage.
So here are these huge machines digging up the earth to make way for an underground parking garage.
Lowell uses "underworld" rather than the word we'd expect: underground. When we read "underworld" we think of graves, or a realm of the dead. Spooky!
Not at all inviting.
Remember he described the aquarium as a "dark downward vegetating kingdom." This definitely resembles that previous description.