The nationalised railways were in their time world leading.
You have got to be joking. What "time" did you have in mind? What form did this leading take?
The Modernisation Plan was probably the single greatest disaster for UK transport policy in at least the last 100 years, as it meant that the Treasury were not willing to let the railway industry near sharp objects for a generation; its failure almost inevitably brought about Beeching, which was another disaster. The Modernisation Plan and its failure were the inevitable consequence of confused, sclerotic central planning by short-term government interests; I hold no brief for the post-war ex-LMS management of BR, and their respective memoirs are self-serving hindsight, but had the railways remained in private hands after the war (ie, the "Big Four") they are highly unlikely to have fucked things up on the monumental scale BR managed 1955 to 1968. They had a good history of long-term planning, and it's telling that BR did nothing in its first thirty years that the LMS hadn't managed to squeeze into the period May 1945 to 1 Jan 1948.
That the Research Arm in Derby did some interesting work on tilt (arguments about BR almost always degenerate into wild claims about the wonders of APT-E) are undermined by the fact that tilt is a dead-end: Shinkansen doesn't need it, TGV doesn't need it, HS2 won't need it, not even ECML needed it. A fantastically complex and expensive solution to a problem you shouldn't need to solve ("how do we make trains run faster when we don't have the political will to fix a few tight corners on WCML around Rugby") brings to mind the old quote about c'est magnifique, mais ce n'est pas la guerre: c'est de la folie. British railways (with a small r) were blighted all through the period 1850 to 1947 by stupid compromises brought about by a refusal to fix civil engineering issues, and APT-E is just the same ludicrous internecine squabbling within the industry with more state money to squander. So the Italians took tilt over and then sold it back to us: that's two countries whose rail policy has been completely fucked for fifty years.