Could Chris Grayling not ask Parliament to legislate to change this if he wanted to? “Pleading” with the railway companies to do it by themselves is hardly likely to fly when it isn’t in their financial interests to change IMO. I have little sympathy for him on this to be honest.
In fairness though, I saw him interviewed and a good point he made, I thought, was that if renationalisation happened things would not be “magically better.” He said that, if it happened, the rail network would be forced to compete for government funds with schools, hospitals, defence etc and would almost inevitably lose out.
As a little aside, I was pleased to see an RMT representative interviewed on Good Morning Britain today and really smiled

when I realised I remembered seeing him interviewed on a BBC “Inside Out” documentary which I saw as a child. Turns out it aired in 1991!

Definitely the same man though.