Tom Anderson wrote:

Cannon Street bridge is 7.1 metres above the high water level of the
Thames [1], perhaps less at the ends. The rail deck will be a little
above that - not more than 9 metres, and probably less. It's about
260 metres from the point where the bridge makes landfall to Cannon
Street itself, where the space occupied by the station ends (ish). If
you hollowed out the existing station and put in a downward-sloping
track at a gradient of 1:30 (generally considered the practical
maximum, i think), you could drop 8.5 metres in that distance, leaving you
at about the level of the
river. You need about four metres over the top of the track for the
train (six if you want OHLE, which we don't). If Cannon Street is
four metres or more above the level of the river, this idea is just
about plausible - at least this far!

In terms of getting to Moorgate, you're then alright, as you have
about 650 metres to run, in which space you can dive another 20
metres, which i think is enough.

You would need to have flat platforms under Walbrook, to replace Cannon
Street Station.