The arguments will run and run. I would take the view that both the road network and the railway system are in need of some infrastructure upgrades, to take into account both present and projected future needs.
The rail network, especially around London, needs some major investment.
http://fullfact.org/factchecks/high_speed_railway_hs2_train_capacity-28742
And the road networks are definitely in need of help. There are still regular tailbacks, queues on jams on the M6 between Birmingham and Manchester; the A14 virtually from the A1 our to the east is often a bottleneck, and I am sure there are many more examples. These are not new issues either; They have been there for decades.
The UKs transport infrastructure is in need of some serious investment. Whether HS2 is the right way to go about improving things is a different story, but I for one think that improving the links between Birmingham, Nottingham, Sheffield, Leeds and Manchester - and of course with London is a good thing.
I used to travel around in Germany quite a lot on the trains, and they seemed to make pretty extensive use of double decker carriages - not something we see here - I imagine this has something to do with tunnels etc? But such initiatives should be given proper consideration....