Bob, I think another key part to your point is transportation en mass 25 years from now will be completely different and I doubt there will be wheels under it! There are many developing technologies, linear induction power and tubes are all on the horizon. Unfortunately as you point out assuming they are ready in 25 years the technology will be out of date and cost over the moon!
I may seem I'm going of topic but believe me it relates to Birmingham's and England's future travel and technology
Back in 75 a engineer from Jaguar told me in the future they would find it easier to train electrical engineers to repair Jaguars than guys like me.
He said we would open the boot pull out a disc and put it in a reader and it would have a record of the cars history
Well he was right but the disc played music and the reader plugs in to the car under the dash.
Now stay with me here because this we can do NOW.
You get home park the car go to bed but the car does a systems check and see's there is a problem
The car communicates with the dealer the system there orders or reserves the parts needed and informs your car.
One two days later the car drives it's self to the dealership the problem gets resolved then drives it's self back home.
These self driving cars why put a steering wheel in there, a lot of cars today that wheel is not connected to the steering, the wheel is there to make us feel safe have you seen how they steer cruise ships.
Back to mass transit what we really want is convenient transportation that's why we own cars, do I think we will be flying round Brum city center no.
I can see in our future small electric cabs like the ones at airports and travel to and from major city's by drones
Then the Eagle and Turn pub and other buildings would be safe and we would not need these massive inferstructure projects.
I think the effective life span is limited of the HS 2 but we can be sure it's coming.