The 175/0's and 175/1's are diagrammed seperately, so on a perfect day certain services are meant to be 2 car and others meant to be 3 car, it's not done randomly. The 1131 is booked for a 2 car. It's normally quite busy and a seat recommendation is recommended. They are almost always displayed the train as planned so long as the train itself is compatible (158s and 175s are interchangeable in terms of seat reservations as seat numbers have been matched up - but this is not the case on 150s). Bear in mind though that you will need to obtain a seat reservation from a station - tickets booked online only normally come with starred out reservations that don't actually reserve a seat.