In message , Roland Perry
writes

I'm astonished that there's so much room! Must have a better look next
time I'm there. All I remember is the loading gauge and a general
feeling of claustrophobia when approaching even in a car, that made me
choose the main tunnel instead.

I think the part that is currently used is only the upper deck shown in
the ATS cross-section (2642mm high) - IIRC it is a couple of metres
above the main roadway, as shown.

It looks as though they are intending to put the bottom pair of tracks
on the lower deck, which was originally the pedestrian route through
from the Bath Road when the tunnel opened in 1955.

--
Paul Terry