A nice feature such as these huge rusty doors would be cool on the set.
Great staircase coming down into the tunnel system!
A set would only need an inch of water in, just to create the reflection. This brickwork is rather too clean and new looking.
A hint of a junction is required for the SWATT team.
Nice tunnel silhouette.
Maybe the stairs in the stadium location could be incorporated into the set as the steps down into the sewer?
Nice layering of brickwork arches would be cheaper to build and give more hiding places but probably too open for the confined action.
Add extra texture of calcium deposits from ceiling and oozing through walls to show it's really old.
Crumbling old brickwork rather than clean fake looking brickwork. This'll make the collapse of the tunnel appear more realistic.