But what do you mean 'upgrade'? The problem we've found is that meaningful upgrades are often as difficult and expensive as full-blown new lines. The Birmingham-Bristol line needs separation between fast and slow services before you can meaningfully talk of speeding up existing fast trains, and the only practical way of doing that would be to build a totally new pair of tracks along most, if not all, of its length. Therefore, you're really talking about a new high speed line as part of an even larger national network.
I think there should definitely be wider consideration of our trunk InterCity network in future. Even if it will take decades to build and no concrete route options have been identified, there's still benefit from a planning perspective. If the original HS2 reports had also considered what to do about the TransPennine corridor (rather than just listing it as a nebulous upgrade programme) then we might have much more clarity on what junctions or passive provision HS2 Phase 2b needs to include.
The right way to do this is to be expansive rather than restrictive. It means seriously considering the future of lines that we're currently in the process of upgrading, like the GWML. I think any Birmingham-Bristol high speed line would definitely need to be considered alongside a new Bristol-London line, as they'd certainly share the expensive infrastructure in and about Bristol and any further extensions into the West Country.
