You miss the point that a quality train service greatly increases the effective size of a locality, thereby greatly expanding access to education and employment options for people of all ages.
It does nothing to affect the effective size of an locality, it simply offers an alternative transport method to other localities, and one which in most cases is less convenient than driving.
Take for example my town in Meath- if the Navan rail line is ever actually delivered that will bring all the Dublin colleges (with the possible exception of UCD) within reasonable daily commuting distances as compared to the hell of trying to do the same distances by bus (a hell I know from experience and will never do again) and the expense and discomfort of driving (again, something I know from personal experience and will never do again). That would I expect greatly increase the likelihood of my child wanting to stay local without drastically restricting their third level options (assuming of course they even want to go to third level).
That will be a 2h trip to Belfield, there are already bus options doing it in less and driving is quicker again. A 4 hour commute is not a good college experience for anyone.
My wife's family is from Virginia, nieces and nephews approaching college age are looking forward to getting away, it's the natural order in quiet locations, teenagers consider them boring. A train service that takes 2 hours to get to Dublin doesn't make them want to stay there.
Most of the people around there already work in Dublin, similarly in Navan, look at the volumes of traffic coming onto the M3 there every morning.
It's similarly difficult to see how, for example, being able to travel the 132km from Letterkenny to Sligo in an hour and 20 minutes by train rather than 2 hours by car would do anything other than improve the attractiveness of Letterkenny as a place to live.
Google tells me the typical drive time is 90 minutes. Regardless, it might influence a few who can't or don't drive and live within a short walk of the station and only want to travel to places within a short walk of Sligo station. The reality is the majority would need to drive to the station or get a lift, then once in Sligo they'd need another transport option to get them to their destination.
I'm from Kilkenny, my elderly parents with their free travel visit Waterford for a day out on occasion, they would never countenance getting the train when the car gets them door to door in less time and with less fuss. I'd consider it a fine place to live now, but I'm old and boring, I couldn't wat to get out when I was young.