The BBC reported on 24th August 2006 that the UK population had exceeded 60 million - see http://news.bbc.co.uk/1/hi/uk/5281360.stm .  The report was from the Office Of National Statistics for the year ended 30th June 2005.

I have looked back - in 30 year increments

33,200,000   1876 (includes Ireland)
43,400,000   1906 (includes Ireland)
47,100,000   1936
54,600,000   1966 Railway modernisation as per the Beeching report
58,200,000   1996 Privatisation
70,000,000   2026 (guestimated as of 30th June)

At Sheffield Park on Saturday, I picked up an old timetable for British Rail's Western Region dated 1965, showing services on lines lost over following years.  Some were - dire - in their frequency, not fast, and looking at the "diagrams" there were a lot of trains busy doing nothing for hours on end.   I find myself wondering with a 28% increase in population since then, and modern environmental concerns at the inefficiency of modern cars for transporting people (not only fuel use, but congestion and the need for road and parking capacity) how different things might have been had the population been so much greater in those days.