People are only going to use the train system where it is convenient and affordable.
Yesterday I needed to get to Stansted for a meeting. I drove to Melton Mowbray station which took 40 mins. It was only £2.50 to park for the day and it's a 2 minute walk from car to platform. The trains both ways were clean, comfortable (I had a reserved seat) and ran precisely on time and what's more the cheap day return fare was £29 for a 2 hr 15 min journey each way. Having arrived at Stansted I was exactly where I needed to be and did not have the hassle of parking the car. Journey time door to door was 3 hours each way - the travel was a thoroughly enjoyable experience whereas driving the 110 miles would have taken just as long, cost a lot more and been far more exhausting.
One aspect which I dont think has been mentioned in this thread is that we need the trains to operate 24/7. The Stansted Express service to London is a classic example. The last train is at midnight and then the next is at 0500. I have seen occasions where the terminal is full of arriving pax but the trains are not running!