Also consideration should be given to the amount of time passengers have to stand for. In the case of Northern Rail any of their diesel trains can be used on a journey of up to 3 hours so removing 20 seats and having 12 passengers standing for 1.5 hours on some services would not be a good move.
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How often does that really happen? Yes we hear about people standing all the way between Liverpool and Manchester on EMT and also between Manchester and Sheffeild also on EMT. But would someone stand between Liverpool and Sheffeild? Unlikely as they would take the seat at Manchester.
I suspect in most cases someone will get a seat after 45 minutes. (which yes is longer than 20 minutes).
Judging by the most overcrowded routes I have used recently I observed the following:
Birmingham - Scotland (although standing until Oxenholme most people standing from Wolverhampton would have got a seat at Preston.
Manchester - Scotland (most people would have got a seat a Preston, not sure how full the rear portion was, I suspect people going to Lancaster use the front protion to get home quicker rather than sit in the Windermere portion).
Manchester - Leeds (Standing until Huddersfield)
I have seen Edinburgh - Glasgow standing all the way when short formed and the same issues on London -Cambridge expresses. But standing 1.5hours on a train, very unlikely. Even XC you can normally move into a seat at a major station.