Just as in life with its many difficult decisions, when entering the train one must choose between sitting on the right side or on the left side. A correct decision will define the quality of the trip from beginning to end, so it's fairly important that a certain amount of deliberation takes place.
Assuming that there are still enough unoccupied places in the current carriage or in one nearby, the choice is based on a couple of non-trivial though important factors.
The right side of the train. This is the sunny side which is nice if you are seeking some extra warmth on days which are not too overcast, say when the temperature is a bit cooler than usual. There might be a slight glare while looking at the beautiful countryside passing by, especially during the fall and winter when the sun drags its feet a bit lower on the horizon. If reading a book or a newspaper, the extra light makes perusing such literature a more pleasant activity, at least for me where clarity and contrast of black on white is important.
The left side of the train. This is the shadowy side which is better if one is trying to read email on a laptop or on exceptionally warm days when a crowded train carriage can cause one to sweat and huff and puff more than usual. While more pleasurable to sit in the shade during the latest heat wave, most others are thinking the same as you. Meaning that the extra mass of human cytoplasm will tend to collect on this side of the train and make any available seating much more cramped and uncomfortable. Especially if that fat lady who always smiles at you comes and sits next to you again.
The main problem for me is trying to remember which is the right side and which is the left side of the train. In the afternoon, the train travels in the opposite direction as in the morning. So remembering is a bit less obvious than just looking at which side of the tracks you are standing on. The same train might stop several meters further up or even at a different platform. Once inside of the train, searching a while for an unoccupied seat, one becomes confused and disoriented. On hot days vertigo might kick up the dust in your head.
If by chance you are rushed and forget to think about right versus left, then the odds remain fifty-fifty anyway. Just don't think and get on with your life like all of the other normal folks sitting around you.