We need to be careful with population density. The north of England has vast areas of land unsuitable for development because its so high above sea level. Its boggy because it has a wet climate. It is dissected by deep curving valleys which makes road and railway building difficult, and its full of factories and housing because there is nowhere else to put them.
So while the ECML and the south end of the WCML are fairly straight and level, the WCML north of Carnforth and the trans-pennine routes are more like Switzerland and Austria with bendy heavily graded railways.
But if you look at a map of England and see the undeveloped areas in the Pennines, you have to remember those areas are 'socking great hills'. So bendy railways in the valleys or expensive base tunnels.
So while the ECML and the south end of the WCML are fairly straight and level, the WCML north of Carnforth and the trans-pennine routes are more like Switzerland and Austria with bendy heavily graded railways.
But if you look at a map of England and see the undeveloped areas in the Pennines, you have to remember those areas are 'socking great hills'. So bendy railways in the valleys or expensive base tunnels.