He is out of bounds now (37)
This is a cryptic one. But it refers, at least on some level, to Colonel Shaw's death, as well as his somewhat unpopular decision at the time to lead an all-black infantry. Colonel Shaw's decision marked a huge historical change as he broke through the boundaries of accepted attitudes toward African Americans.
The stone statues of the abstract Union Soldier grow slimmer and younger each year— (45–46)
The statues haven't physically undergone any change, so don't let your mind wander to Benjamin Button or anything. The transformation comes in the fact that they're being forgotten by the people of Boston today.
The ditch is nearer. (53)
We're moving toward our own downfall? Digging our own graves? This one is open to some interpretation, but it's a negative transformation, for sure. You're gonna want to steer clear of the ditch.