I see a theme developing here. Ex emergency services and forces personnel tend to become train drivers. I was awaiting a start date from Merseyside Police and it still hasn't come along. Im also thinking that maybe being 23 is too young to be a driver seeing as I didn't get past shortlisting with Arriva Trains Wales.
Please don't see that theme as anything near the norm.
In all honesty I'd say pretty much nothing. There some common advice running on this forum that getting some relevant experience and some railways experience before applying for a Train Driver job may be beneficial. In my experience, just apply. The whole application process is the same for whoever and whatever your background is. The only time you may have an advantage is either through internal application and at interview stage. However, in those it is more about life experience than career background.
Don't hesitate, just apply.