Sorry, it seems I wasn't clear. I agree that's an obvious non-starter for all sorts of reasons. What I meant was TfGM specifying the elements that operate within GM. A purely hypothetical example might perhaps be that all services within GM must carry a second member of staff on board; or specifying certain frequencies on certain routes. Without wanting to put words into the Mayor's mouth, I think that's the kind of thing we're talking about.
That sort of thing would only work if TfGM were happy to incur the costs of the extra staffing/services or fare increases to pay for them.
Burnham is typical of politicians who make demands but don't have to pay for them.
The devolution of finances is one of the tricky things the government is very reluctant to do, though a formula has been found for Scotland and Wales.
And if northern residents don't want GM dictating their services, it's what already happens with Transport for Wales services in Cheshire and elsewhere.
Whether they like it or not, Cardiff is in charge of a chunk of their rail services, even those at GM stations.