The renationalisation of British railways seems to be the new public fetish; we have a short collective memory it would seem.
First and foremost, the system we have today isn't a proper market in railways. It's a system known as "franchising", where operating companies bid for use of the railways. This, essentially, creates concentrated monopolies which, when coupled with the bidding mechanism, raises prices and then the government makes everybody bear part of the cost with fare subsidies.
But what does the graph tell us? Even a bad system of privatisation is better than nationalisation. During the initial development of the railways from around 1830 to 1913, the system was properly privatised and construction was done mainly with private capital. The government took effectual control during the First World War, and then in 1923 David Lloyd George was dissatisfied with the high competition and low profits of the private railways, and so consolidated them into regional monopolies known as the Big Four.
They performed very poorly, and then following world war two the entire system was nationalised under British Rail. In 1995, the Major government introduced the current system and by 2010 it had made up the lost ground of the previous 80 years.