There a system of joists that run horizontally and they support a form which will support the roof slab construction. At this point you're just about home free. You're casting the roof of the tunnel and you're very near completion of the work.
Here's a completed section of the tunnel. It looks reasonably dry. You can assume you're always going to get some seepage and there needs to be a positive drainage system with pumps in the new facility. If you've done your work carefully, if you've paid enough attention to the waterproofing, that seepage should be minimum and easily handled by sump pumps.
Now you really are just about finished. The tunnel has been backfilled. It appears as if some waterproofing has been added to the existing tunnel where it might have been damaged. The waterproofing here is being restored, and it is joined here with the existing waterproofing. I would like to point out that you're joining together modern-day materials and materials which go back 100 plus years. You need to discuss this with your manufacturer and your installer, because very often these materials are incompatible. You need to be very careful about choosing means and methods and materials which will allow the new material to properly attach to the existing material.
Just when you thought you were finished, it's utilities again. The utilities which have been uncovered and supported during the excavation, now have to be restored in permanent conduit and perhaps encased in concrete. This is relatively easy in an area like this, where there is no decking over the excavation. Where decking exists and the utilities are hanging from the decking, there is a real challenge restoring them to their permanent location and permanent condition, and then detaching them from the decking. You're doing that work right underneath the decking with limited access, limited space to work. This is an important part of the job. The tunnel is finished. Yes, that was the easy part, relatively easy, but restoring the utilities represents another challenge.