Before the electrification of the West Coast Main Line, there were two routes from London to Birmingham, with little to choose between them. The direct Great Western route, which ran through High Wycombe and Bicester, was then more or less abandoned as a through service, with the section from High Wycombe to Aynho Junction being reduced to single track.
Since privatisation, the route has been upgraded and converted back to double track. Earlier this year Chiltern Railways introduced a through service from Marylebone to Birmingham, and there is even a suggestion to electrify the route.
Unfortunately, further development is going to be limited by restrictions at Marylebone, which in the space of twenty years has been transformed from a sleepy backwater to a very busy station with a large volume of local traffic.
There is, however, another route into London which is scarcely used - the former main line into Paddington which branches off the Great Western route to the west at Old Oak Common and then run alongside the Central Line to Ruislip; the line from Marylebone joins this line at Northolt Junction.
At present it would not be possible to increase the amount of traffic on this route due to the volume of traffic using Paddington. If, on the other hand, the Heathrow Express were removed and given an underground terminus of its own, as part of the Crossrail project, this would reinstate the entire route and double the capacity between London and Birmingham.