You mean this one
http://wmbusphotos.com/Trains/class172/172217.html
I don't really know! I had just got off my train arond 9am on 17th March (a Saturday) and saw it pulling in on Platform 12 with passengers on. I went over to see what it was doing and get the photo, but all screens on 12 were not showing any departures and the crew just left it there