Which advocacy approach would be most effective

Do I join rail pax assn oe ESPA OT form my own group - like “Rail Passengers Union?

You should definitely join the Rail Passengers Association https://www.railpassengers.org/ as well as the local and regional organizations in your area, listed here: https://www.railpassengers.org/all-aboard/local-organizations/

As former chairman of the Virginia Assn. of Railway Patrons (merged with Virginians for High Speed Rail almost two years ago), I say join an existing group. Starting, building, and running a rail advocacy group is a lot of work. If you have people interested in and willing to work toward specific goals that are not the focus of another group, it may be worthwhile starting something new. For example, the Piedmont Rail Coalition (I think it was called) was instrumental in getting service added to the Washington-Charlottesville line (besides the Crescent and Cardinal, which was all that existed when the group started). The Virginia Assn. of Railway Patrons and Virginians for High Speed Rail were in favor of what the Piedmont group was doing, but the Piedmont group addressed a particular service and was successful. If there's no need for a new group to address your rail service concerns, I recommend that you join an existing group and pitch in to help.