Annoyingly, it's purely down to a lack of investment. The failed experiment of a privatised railway should be binned off, get the trains back off the leasing companies into public ownership, and stop paying the likes of First and Go Ahead to run the railways badly. All this money going to investors in dividends could be spent on maintaining and investing. Not to mention the fact that HS2 failing to come through is only going to make things worse on the already overcrowded mainlines.