Disclaimer - I say this as an economic migrant who moved from a Scottish town to an English city - maybe I'll be included in the statistics if the SNP get their way...
My original heading for this thread was "Overcrowding - a reason why never mentioned" was changed by the moderator as it was thought that my question appeared too vague. By my original title, I just wanted to ask why it was that the influx of large numbers of migrants in recent years parallels the massive increase in numbers of people using public transport
Okay, I'll take you at face value and ignore any loaded assumptions that could be made in a thread like this.
Taking Wikipedia's figures (
https://en.wikipedia.org/wiki/Demography_of_the_United_Kingdom#UK_Population_change_over_time),
the UK's population increased from 50,225,000 in 1951 to 56,357,000 in 1981.
During that time, rail numbers went down significantly. If "more immigrants cause more train journeys" then how come train journeys reduced so much despite over six million extra people living here in 1981 than in 1951?
(people will blame Beeching for the drop in passenger numbers, though they were going down steadily before his report, whilst population numbers were increasing)
You could even argue that the "influx" in recent years is required to pay for the current generation of pensioners (annual deaths were higher in 1901 when the population was 38m than they are with 60m+ in the country - fertility rates are significantly lower than the 1960s - improvements in life expectancy mean that the big change in population is the over-65s - one in six people is over that age - which means we probably need a few more immigrants of working age to pay for their pensions

)
Well, firstly, the quoted figure for numbers living in the UK not born here is fairly meaningless, as it ignores emigration which reduces the net effect
Good point.
The hysteria about "EIGHT MILLION FOREIGNERS" living in the UK that got mega coverage in the populist papers recently is irrelevant if you don't deal with the numbers born in the UK who've moved away since birth (I know a few who've moved to Canada/ New Zealand/ India/ Australia/ US/ Germany).
The ethnic mix of rail passengers isn't much of a guide given that many non-white people will have been born in the UK.
True - something that makes "standing on a platform and assessing colours of faces" meaningless - if you're born in Britain then you are British as far as I am concerned (regardless of skin colour).
Then again, I think of people like Chris Froome (born Nairobi) as "British" - don't want anyone claiming that was a Kenyan victory in the Tour de France
... and people retiring in greater numbers than previously (baby boomers), and living longer and staying more active once retired.
Huge percentages of my fellow rail passengers are Senior Railcard holders, generally travelling for something to do! (I include myself in that

.)
Fair point - that demographic has grown significantly - and richer/ more mobile than previous generations of pensioners
Indeed. General affluence.
e.g. it seems impossible to board a Friday or Saturday train to the likes of Liverpool, Manchester or Blackpool without encountering at least one stag/ hen party. A market which didn't exist 30 years ago, when a stag night meant a tour of local pubs. Usually.
True.
A lot of "special occasions" are becoming "the norm" - e.g. the days when a trip to London was something to write home about - the kind of leisure market that you mention above is becoming something some people might even do every week.
Wouldn't the congestion charge, fuel prices, housing prices in major urban areas expansion of concessionary travel schemes to include rail be a major driving force behind overcrowding.
True - much more complicated than simply "blaming" one factor