The 90 isn’t going to be powering the train - it is just a vehicle with an instrumented pantograph. The usual OHLE measuring vehicles (Mentor + the equivalent in the NMT) are our of action at present; so NR have improvised. The 67s will power the train. On the off topic point of whether the GW electrification is insulated from the rest of the network - there is no electrified connection to the rest of the railway from the GW at present. While there is a connection from North Pole depot to the West London Line, it is no longer electrified; and even when it was it was 750V DC 3rd rail, not 25kV AC OHLE.