I avoid the rush hour while commuting. Since I stay far away from where I work, I have to catch a very early train in order to be at work on time. Then there are days where you tend to miss the your original schedule and, as a result, is caught up in the rush hour. This occasion turns out the day on which you don't get a seat, and you end up standing till your destination. For commuters to London from Reading, this is a daily phenomenon. But I travel in the opposite direction away from the regular traffic.
During the rush hour, most of the seats are reserved. Just like in India, there are unreserved compartments although I have not figured out how to identify one. In reality, all the seats might not be taken. The reserved seats have a piece of paper sticking out of the top of the headrest. This piece of paper indicates the length of reservation with respect to the from-station and the to-station. You can still squat here, but you will have to vacate when the rightful owner turns up.
On the day when I took this picture, most of the seats were occupied, and these stubs were sticking out of most of the seats. I found a seat after the first stop.