This all begs the question of perspectives. On most routes there will be many times of the day when trains run almost empty and few travellers note or comment on that. A few trains will be rammed full for parts of the journey and the crowded masses will shout out. That's only natural. We all know there's a big tidal flow element, particularly at commuting times. That's relatively easy (or was pre-Covid) to plan around.
Surges in leisure demand are much harder to work around, not least because they tend to be irregular, are greatly driven by specific events and by the weather.
Add to the mix all sorts of totally unpredictable railway hazards causing delays and cancellations with further knock-on delays to many other services. A planned for load can very quickly be overwhelmed by bumped and diverted passengers from other disrupted services.
Which begs a question. What levels of loading do passengers prefer? 100% being all seats taken
Under 25% some may feel anxious about other passengers near them, but most welcome the space.
Up to 50% is comfortable with all able to get a choice of single seats without sharing a 2 (or 3) with anyone else.
At 75% most will still have a selection of single seats available because many couples will be sitting together.
Over 75% gets more delicate. Do we ask that person with a bag on the seat to move it so we can sit next to them? Take their feet off the seat? Do we want to sit next to an old/young man/woman? Maybe opt to stand?
By 90% quite a few may be standing from choice.
By 100% there'll still be a few empty seats as above but new boarders will probably stand.
By 110% pressure will have forced most bags off seats, although there's always one.....
By this time 'the railway' will accept the service is crowded.
By 150%, depending on rolling stock and distribution down the train, passengers may be unable to board.
Crowding levels are liable to fluctuate greatly along long rural/regional routes. I'm thinking of Norwich - Liverpool and Manchester - Milford Haven as examples (both between 250 and 300 miles) although there are many others. CrossCountry may not strictly count as rural/regional but it certainly has the longest routes made up of overlapping traffic flows.
People will be attracted to trains that regularly run to timetable with loadings below 75%. Once short forms, cancellations and late running trains are factored in that desirability starts to evaporate. My experience on the three services above suggests that for sections overcrowding is likely to be deterring further custom.
What to do about it? Maybe lots more fresh air trundling around the rails in empty trains causing wear and tear on tracks and rolling stock with more staff to maintain both. Or splitting trains and causing more interchange disruption for through journeys. Other threads have already aired these views.
On my line just running all the scheduled trains at full planned length would be a very good way to eliminate congestion.