I can't speak specifically for train driving jobs, but some background...
The timescale for an offence to become 'spent' depends on the sentence. Something involving a prison sentence of more than 2 1/2 years is never spent, a caution is spent immediately, and it's a sliding scale between the two. More
here.
Other than people who are formally disqualified from working with children and / or vulnerable people, it is not the role of CRB / DBS to say to an employer "yes you may employ / no you may not employ this person" - it is their role to provide information and let the employer make a decision.
The jobs that are exempt from the Rehabilitation of Offenders Act (i.e. where you have to declare spent convictions) are defined in law / guidance - an employer can't just decide to exempt a position because they feel like it.
If the application asks you to declare unspent convictions, and this is a spent conviction, you don't have to declare it.
If the application says you must declare spent convictions, then it's going to put you in a more difficult position if you don't declare something that later shows up on a disclosure than if you're upfront about it.
Employers are not supposed (with a possible very few exceptions - although I think you can even join the police now if you've got a minor conviction or two) to reject just because of a past conviction - but they may want to discuss it with you, and to be confident that you're now a reformed character / it was a one-off / youthful indiscretion.