I tried to scope out the faults along your tunnel route using the online maps - it looks like it would parallel the active ones. No slip galleries required? Supposedly the BART Eastbay hills tunnel has already used up its creep allowance and needs redone.

The downside of multi-use would be if, say, there were a fire in the highway bore that presumably would close the rail line as well.

The locals are game for this tunnel scheme? Seems they did not much care for PB's various plans.