Good question Will!
The EPA regulations (attached), from what I can tell, do not address anything other than Class, 1,2 and 3 railroads and any commercial operation that hauls passengers or freight. Only the FRA Safety Regulations, as outlined in Appendix A of 49 CFR Part 209 address Tourist, Scenic and Historic Railroad,on or off the general railroad system. This only applies to FRA regulations.
With that said, the EPA regulations are very voluminous and they do address "small business" throughout. There may be some kind of exception for certain smaller railroads, but it is hard for me to figure that out, at least at first glance.
Mike
98-7769.pdf