If you get on anywhere near rush hour (esp in/around London), youll find the commuters have already grouped themselves into perfectly equi-distant little groups anyway!
Quite interesting to see. Bit sad to think that Ill likely be one of them some day...
We get that in Cardiff as well at rush hour.
At rush hour the aim changes from getting a seat, to getting the most comfortable standing position. Providing you have a wall to rest against you can journey quite far in some sort of comfort.
Typical favourites are the door areas however theres the middle link between two carriages thats an often neglected area... Baggage racks in the older carriages can also make for good seats.
Being a fat bap I tend to look for places where I can "fit into" and get out of the way of commuters until it comes to my stop, and luckily many people forget that when getting on at my station... all the other stops are on the other side of the train, so they dont move over, meaning you have a good 30 seconds or so to pick your spot.
If on a crammed train, dont be afraid to open the doors even when noone wants to get off, take a step outside cool down (as trains tend to get stuffy) and get some fresh air. As the beepers go jump back on and away you go.
I regularly take advantage of the above point in the summer. With no room to move, a heavy coat, and a bag on me its my saviour
Noticed quite a few other people taking the opportunity to do it as well whenever I did it.