According to my research this should be a 3 SUB but clearly is is 4 cars.
This shows the unit at a stand-still just before Lordship Lane station on the Crystal Palace (HL) branch.
It is not running wrong line but rather an ‘official’ photo where presumably the driver has walked from his cab at the front to the rear. Note the tail lamp. Bags of time, the weekend service was hourly. Or possibly there was another crew member already in the cab.
Photo taken from Cox’s Walk footbridge over the track.
Photo: M. Gaywood/Rail Archive Stephenson