If anyone has played Cities in Motion, I think that shows pretty much how the new tunnels will work. The current in game placement and features of road and rail above ground is very similar to that, so it makes sense that it will follow the same model. No, I don't think we will see Metros being connected to the surface in that particular patch as the subway tunnels are very simplistic and you can't even change their depth manually.
For those who want things like cuttings for sunken roads and rails, you can do that NOW! Just get the terraforming mod, you can carve out what you like then!
I must say, it's good to hear that they would rather get the main bugs sorted first, most devs would put them asside and release new features like tunnels to try and take attention AWAY from present bugs, nice!