Not having seen it, I'm going to guess it's a substation. That sounds exactly like they did on the land that used to be the Fairfield paper siding. More trains running at Westall mean they need more overhead capacity.
From Memory, There's a substation very close to there... Perhaps they're expanding it.
Somewhat Relevant, I notice they installed a big chunky peice of metal (I assume it's a transformer/inverter) at Altona Jn in the substation that had the giant pink vibrator painted on the side.
I'm not so sure the new building at Springvale is going to be a substation. There is already a substation at Westall up from the station and I think there is already one at Springvale as well. I know there is a works depot at Springvale. All the pipes that have been laid through the carparks seem to be emerging from the ground in the middle of this project. Could a new signal box be out of the question or would they just upgrade the one that is already in the Springvale up station building?