Different RR's use different rules. Mainline RR's use some form of the General Code Of Operating Rules. Many Tourist RR's use their own rule book, which is based on rule books used in the past. Years ago, when I wrote the C&TS rule book, it was based on the old Consolidated Code of Operating Rules used by the D&RGW and a bit of the General Code added where it seemed appropriate. Since then the Rule Book has been re-written, and I don't know any of the particulars.
When I took over as manager of the Texas State RR, they had used the previous operators rulebook - which was based on the Durango & Silverton's, and were pretty laid back in their compliance. Iowa Pacific required TSR to go to the General Code of Operating Rules, which was real struggle from some employees who hadn't had any kind of classroom experience since they left high school.
The Strasburg RR uses it's own rule book, and it is probably one of the more "throwback" rulebooks I've seen in a long time. Nothing like requiring your employees to have common sense and situational awareness drilled into their heads.