You may be interested to read in the Class 777 Wikipedia entry that:
"Because current regulatory policy makes it unlikely that future extensions of Merseyrail's unshielded third rail traction power supply will be approved, Class 777 units will be delivered with provision for the future installation of 25 kV 50 Hz AC overhead line traction equipment". Another article I found elsewhere states that Class 777s will be "prepared for later retrofit of ETCS and 25 kV power supply equipment".
There is a clue to the overhead wires coming at some point in the photo, although I wouldn't expect anyone to know that's what it is. The two square signs in the distance, one of which is above the orange-clad P-way man on the right, are "advance traction changeover" signs, warning the driver that a change of traction is coming up. Beyond the square sign on the left, one can see the larger "pan down" sign (it looks like a white rectangle on a black background from this distance). These signs would not be here if there weren't any overhead wires, or if none were planned to be installed.