Thanks, Tom, for the hint.
I didn't find anything at the Historic Aerials Site, so decided to go to
USGS.
The 1941
Guasti And Vicinity topo map shows PE's Etiwanda Depot near the right edge, about 25% down from the top.
The map shows the single-track mainline, on the south side of the depot, splitting into three parallel tracks on one side of the depot and returning to a single track on the other side. A short spur runs from the track nearest the the depot to the east side of the depot. About 600 feet east of the depot, next to the track, is a large square indicating a building about 250-300 feet on a side, having a much larger footprint than the depot building itself. This may be the the substation the original poster asked about.