I think that there's probably scope for increasing train frequencies too, but as I understand it that would require a thorough overhaul of the signalling and train management technology. I'm sure Bob would know far more about this.
There are all sorts of things to take into the equation. Line speed, gradient, braking distance, length of train, frequencies of train required etc. We use a fixed block system - one train, one time, one block. The continent and other places use what is called moving block which I am not completely familiar with. This is not compatible with fixed block so would require a whole new signalling system.
We use 4 aspect signals in crowded areas, Red, Yellow, Double Yellow and Green. There was a trial of Flashing Green but it was not continued.
