I think an important point is being missed - the discussion is only considering what happens when you sit next to one infectious person for either 1 hour or 2 hours. By changing trains and sitting next to 2 people, you double the chance that one of them is infectious.

If we take the probability that a random person in the UK is infectious as 1 in 1000 (not far off the ONS estimates), and you sit next to 1 unknown person for 2 hours, then using this paper's figures your chance of infection is 1/1000 x 6.1% = 0.0061% (if I got the right number of zeroes). If you sit next to 2 unknown people for 1 hour each, then your chance of infection is 2/1000 x 7% = 0.007%, that is, slightly higher.

Regardless of whether this paper's exact figures apply in this situation or not, this is roughly the outcome I would expect. If you sit next to someone for 1 hour, and don't get infected, then the reason why you didn't get infected - they are not infectious, or they are breathing out of the other side of their face, or they have an effective face covering or whatever, has a good chance of continuing so your chance of being infected in a second hour with the same person is slightly less. On the other hand, if you do get infected in the first hour, then you can't get infected again in the second hour.