Again, does Switzerland really want to become one big mega city? It is up for the Swiss to decide, but a huge commute draw between Bern and Zurich (or even Bern and Lausanne) would lead to something more like Japan.
The necessary tunnels for Basel would probably cost less than this:
https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel
But Bern<>Zurich is the SBB's bottleneck.