Agreed that east-west train lines are no longer useable in any meaningful sense. If you're committed to using the car, why not drive Cambridge to Milton Keynes, and then take the train from there? This gives you lots of options both for fast and not-so-fast trains to Mordor Central.
Blame Beeching for shutting the Oxford-Cambridge line setting up the accounting rules that made the Oxford-Cambridge line (one of his personal favourites) unviable.
