If the experience of our current railway organisation (for want of a better term) doesn't convince you, I'm not sure what would. But here goes...
The fact that pretty much every railway in the world is 'integrated' in the sense of the track, trains, signalling, stations etc. coming under the same organisation.
The fact that there is a continued attempt to have a 'guiding mind' for the UK railways.
The fact that London Underground has been reintegrated after the disastrous PPP experiment.