How do modern high-density commuter trains compare, i.e. 378s, LU S class. I know that the question was for trains with the most passengers but a fully loaded wide through-gangway train probably exceeds the 12-car outer suburbans per linear train length even though passengers would be more comfortable.
This will become apprent when the real high density outers arrive, i.e. the 700s and 345s. The 700s may be expected to carry 1500-1700 passengers in the peaks. Then the problem shifts to platforms and station access handling their ingress and egress.
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Isn't PIXC* the measurement?
Passengers in excess of capacity*.
Train length isn't necessarily a factor.
*Google either of those for some further information.
PIXC is a DfT figure to address perceived passenger comfort expectations, (presumably for use by politicians) and is compared to the TOCs franchise requirements.
Where a service is infrastructure limited, it is more an indication of train types being suitable for the passenger flows.