Whilst strict uniform policy is fine for paid staff, I've never known any volenteer work with a uniform policy anything beyond what is reasonable for safety and reasonable appearance. In recent years I've volenteered at 4 places (2 transport, 2 none transport) and the only policy any had on clothing was in relation to safety clothes for certain jobs, and in a lot of cases they were provided along with other safety equipment. They also expected anyone customer facing to be reasonably smart.
I know both in both the transport groups there has been mention of authentic clothing before, but oviously the issue of cost, especially for volenteers, comes up, as does practicality and safety (there are many situations where modern clothing is more practical and safe than what would be authentic). There is also the fact that unless you are portraying one particularly era then uniforms are unlikely to be totally correct anyway.
To be honest it only pleases the purists and rivet counters anyway (who are also the ones who would pick on the fact that the particular train or vehicle is in 1960s condition but the staff have uniforms in the style worn in the 50s as they changed the buttons in 1958). Most people won't care.