Extra revenue requires either more passengers or higher fares. I assume higher fares are not wanted, except perhaps at the margins where cheap tickets are leading to overcrowding on certain services. More passengers is a better option as it also contributes to national policy of encouraging more connectivity by more sustainable modes.
Rail has three major passenger markets: intercity travel, London commuting and regional services (everything else)... .
An insightful analysis, and I agree that it is in the latter where there is most scope for growth.
On this front for travel to grow, rail needs to become more competitively priced for journeys to be increased. At the moment, my experience is that rail is not well priced for regional rail travel outside of PTE areas, particularly when you more than one person.
Case study: I live in Durham and want a day out in York with my wife. Presuming for a moment that I'm not a train enthusiast and that I'm not interested in green issues, at the moment I don't see how rail is competitive. The cheapest flexible fare is £47.50; the cheapest Advances can make the return £18, but this involves booking 3 months ahead and travelling at anti-social hours, and all of this with a Railcard discount included.
By contrast, our fuel for the day would cost us around £20-£23 depending on fuel price. Even if we factor in running costs of the car (which we'd be incurring anyway if it were sat on the drive) and parking (though frankly I'm sure we'd find somewhere free and walk), only the cheapest Advances, with their associated loss of flexibility and the danger that we'll change our plans, are even slightly competitive.
Train fares are competitive for longer distances where journeys are more likely to be planned in advance and travelling times are much better and for travel within urban areas. Possible growth, I think, is in that intercity market of journeys of 30-120 minutes.