I think it has always been the case that the majority of volunteers had no railway or heavy engineering background, and could probably have been counted as "civil servants/paper-shufflers/shiny seat brigade/bean counters" - also a large number of students and teachers - just look back at pictures of those tweed jacketed and anorak wearing volunteers of the 50s and 60s who obviously didn't have a clue. The important thing is that they were welcomed, and taught what to do by those with the relevant skills and experience.
In the past, those with the relevant experience probably got it from working for British Rail or industries with the relevant skills. Increasingly, they are those who have acquired the experience and skills from the heritage railway sector itself. When I was a new volunteer, the experienced staff took time and trouble to train me up. Now it is my turn to pay that back, by training up the next generation of volunteers. Unfortunately there are some who seem to resent having inexperienced trainees foisted on them, and worse still, make it evident to the trainees. You only have to read disparaging terms such as "shiney seat brigade" and "unless you are used to dirty finger nails then you are of limited use" to see the kind of attitude I am talking of. New volunteers need to be valued for what they can do, not denigrated for what they can't do.
Yes, there will always be a level of drop-out, as people find that they don't like it, or other life priorities take over. But that has always been the case, for as long as I have been volunteering. The important thing is to treat everyone as a potential gem, not to treat everyone as a potential drop-out, because that will become self-fulfilling.
And a large part of welcoming new volunteers is the out-of-hours socialising. If everyone just disappears as soon as their shift finishes, that is not going to be very encouraging to new volunteers. The new volunteers need to be welcomed into the social side of activities too. They need to be made to feel part of the group, not outsiders.