Hi El,
Sure! Put in more new ties than are required per these standards. It is all determined by money, but more new ties will reduce the stress and compression on the adjacent ties to make them last longer, reducing later costs. Most narrow gauge tourist railroads would fit Class 6 track stardards if you just look at ties. Somewhat of a waste as they are not going get close to running the ng trains up to 110 MPH.
The thought of having track standards to protect you from a lawsuit is in reverse of the best method. With no stardards, it would require the suing party to prove any defect and the entire dynantics of narrow gauge trains system - a procedure that took a decade or more for Standard Gauge railroad where there were many more skilled engineers to develop the facts. It might take a century for any lawyer to prove this fact, and the tourist railroad would moved on into a next century of service by then. Publish nothing and truely claim you run a safe operation!!! Sh-- happens! Grade crossing are a biggest threat.