It WILL NOT give your "railway skills" that can transfer onto the mainline network.
What BGM has posted above is very true about general workplace skills. For a school lever, some of the experiences you gain, could give you a head start in the job market. But make sure that you are working for one of the more professionally run outfits, that are proper businesses, and are run as such. Moors, WSR, Ffestiniog, etc.
Volunteering is something I've done since I was at school. It certainly gave me something to talk about in post uni job interviews, as could show I had been given responsibility, etc. by a credible organisation. When it came to applying for the mainline railway some years later, I just slipped it into the hobbies and interests section of the CV. When doing my Structured Interview for driving, I was asked not to refer to it, but to rely on my "real" work experience. I let them bring it up in the employer's interview. A bit of friendly questioning, making sure that I recognised the differences between preservation and mainline work, but an opportunity for me to show that I was applying for something that I genuinely wanted to do.
