There's just enough room for you to squeeze between the two subway cars. The chest is behind them and to the left.
After flooding the area, climb up to the pipe opposite the chest, then glide across the water to collect the shard.
Wrath shard 2
The chest is in the same room as the lever you need to turn to proceed on the main path. There's also a chest of souls in this room to collect before you swim back!
Dive all the way down to the bottom of the pool of water to collect the artifact.
Artifact: Soldier 2
On the third lift (in the room with the three lifts puzzle) look for the alcove in the wall containing the Soldier artifact.
This artifact can only be reached after flooding the lower chambers with the giant chunk of rock. You'll have to swim down to it.