indeed as Dave says. If you want to spot it or bash it, then Reading is the best place but you might not be guaranteed to see it as it might not be out at all that day, or it might be on one of those diagrams that does a Swansea to Paddington in the early morning then goes to Old Oak to sit there all day (more likely for exams)....if such a diagram still runs nowadays but I believe there are still a couple of one way only diagrams.