NR doesn't have a standards suite for "tram electrification", though it does for 25kVAC fixed tension trolley wires, but the amount of work required for that and for "full" electrification wouldn't be fundamentally different. Are you suggesting that 750VDC overheads are the future?
I was referring to the disaster that is the Rotherham Tram Train trial, where Network Rail decided on insisting on future conversion capability to 25kVac - despite the fact that the section of track to be utilised in the trial is highly unlikely to ever have a business case for electrification, at least not inside the 30 year horizon.
It is a freight line with minor diversionary use that is north of Sheffield and thus would not be included in any likely MML scheme, even if the whole-MML scheme wasn't dead.
This forced massive delays and a lot of very expensive, and totally unnecessary design work.
Tram electrification is unfortunately the future given that 25kVac electrification is not going anywhere any time soon, especially with the clearance requirements, and it is a readily available technology that can be deployed today.
Given that almost all high traffic lines have been electrified already - we are left with routes where tram-train style operation would fit quite well, at which point 750Vdc overheads become a reasonable proposition.
Although personally I would prefer 1500Vdc bottom-contact third rail.