Think this has been done before

[Ed]

https://www.bbc.co.uk/news/uk-politics-68889345

Bring back BR Blue 



Ed

[PLD]

So cutting through the Party-Political mud slinging, Labour say they will do what the Conservatives had already proposed... 
I.e. as franchises come up for renewal, the in-house DfT operator will be the preferred/default option rather than the 'last resort'... The main difference would seem to be in the closeness of integration of operators with network rail (already Govt owned).
Either way it's a gradual, progressive shift, not a 'big bang' "Nationalisation" and for the passenger won't look much different in the short term to a change between two private operators.