So, several people, including myself have no problems NOT melting the plastic. I've been doing it since at least 2009 and have never had this issue, I have about 850 feet of track with many cuts, and it is all stainless steel, which takes much longer to cut and heats more.
So, what tool and what specific blade did you use that indeed melted the ties? (based on your answers that you did cut rail with an abrasive cutoff wheel and you did melt plastic)
I'd like to know what combination you used that failed and be sure to avoid that.
Greg