The data is from Australia but it won't be any different in the UK:
In 2013–14, there were 178.5 billion passenger kilometres travelled on capital city roads in Australia and 12.6 billion passenger kilometres travelled on urban rail networks. I’ve written before that this share is unlikely to change for the simple fact that only around 10% of metropolitan wide jobs are based in central business districts of our major cities. Agreed, it’s an important 10% for public transport because PT best serves a highly centralized workforce as you find in CBDs. Commuter rail in particular relies on a ‘hub and spoke’ model, mainly designed to ferry people from into and out of CBDs.
Let's develop transport policies that actually respond to the challenge rather than direct investment on the basis of having had a train set as a kid.