[lukasm]

Sure, but the problem is with tube. It already works on its limits and higher destiny would require better city transport.

[eru]

Higher residential density wouldn't add that much strain---the destinations (ie workplaces) still have the same density, it's just that commuters don't travel as far on the tube.

So you could make the system more intense, and less extense; concentrate the investments in tube upgrades on the centres.