Well it certainly hasn't been true of recent journeys I have made from Yorkshire, travelling directly to the airport or changing at Piccadilly. And seeing the 'MAN' tags on suitcases in trains heading east over the Pennines is a very common sight.
And how many suitcases do you see on each train?
100?
The seating capacity of even a 3 coach 185 is on order of 180.
And that is with all the standing room.
The seating capacity of one of the Northern trains is much greater.
A half hourly service not useful? Sorry but for many parts that is a very common pattern, try travelling down the Aire Valley to Leeds some time, with the exception of a couple of extras in the high peaks that is the usual service no matter how busy the train.
The Aire Valley is not run through a completely built up and urbanised corridor is it?
Would half hourly be suitable on the Victoria Line?
Or the inner sections of Merseyrail?
And are there five to nine trains per hour running through the stations on the Aire Valley with plenty of additional space?
Urban railways need much higher than half hourly frequency to be attractive.
Now compare that figure with the 1999/2000 (1.835 million), or even more recently 2014/2015 (3.460 million). Then consider that potentially an additional 40% could be using the services in the next two to three years. Clearly there is growth in the flows, and clearly there will be more to come.
Then consider that almost all the trains on that corridor are well under 100m length.
There is no capacity shortage on the line at all.
Well if we base the evidence on you occasionally looking out of the tearoom window.... However that really isn't much an accurate way to measure it.
Even the passenger stats show low loadings on the trains to the airport.
There nine trains per hour to the airport from Picadilly, which means that the 6350 people travelling to the airport are spread over at least 100 trains, and probably many more.
So on average something like 60 people per train.
When even a Class 185 has a seating capacity of 180, that is
nothing
I don't have any, but Manchester does. Not that you'd know it from the incessant wailing from Mancunians. Seriously, try commuting elsewhere for any length of time and tell me that Manchester has it worse. You've got an ideal corridor to have a fast tram route into the centre from the south central suburbs, which could run at high frequency benefiting far more people than a handful of stations could. You've got a high profile mayor constantly wanting more, and you've got money flowing in. Frankly I am amazed this is even a point of contention.
It's not ideal without substantial amounts of tunnelling.
Having 80% of a good corridor does not mean that the corridor is cheap, the last 20% is nightmarish.
I've been trying to spec reasonable solutions for years and I still can't come up with anything that would ever get funded.
Four and a half million a "relatively small number"?
It is for a station receiving the shear number of trains that Manchester Airport is given, yes.
A number that dwarfs the number using the rest of the line substantially, yet these stations need S-Bahn capacities? Again, I struggle to understand what is so much more important about this small section of line than so of many others, especially given that for some of the areas there are more than the single line to use, and as you yourself admit one of the busiest bus corridors in Europe. But the Styal Line needs more? Hmmm.
Because where else is there a line full of a succession of very lightly loaded trains running through already existing stations that could handle a large fraction of the demand?
EDIT:
Realtime trains puts it at 134 trains starting at the Airport in a given 24 hour period.
6350/134 gives us an average occupancy of 47.....
40% increase in passenger numbers takes us to a total of 65.....
And that assumes that none of the passengers travel to the airport on Northern trains to Crewe..... so it's actually lower than that.
(310 calls in a 24 hour period, so 155 trains total. Which is an average of 41/57)