In the broadest terms, ice reefers began being replaced in earnest starting in the late 1950's and early 1960's and you could say they were mostly gone from the rails by the end of the 1970's.
From memory, early mechancial reefers were being built in the 1950's, but they really took off in the 1960's as a better, more economical, lower maintenence way of shipping produce and products requiring refrigeration. For example, the PFE fleet grew with the 57' R-15-xx and R-20-xx reefers being built in the mid-1960's and into the early 1970's. There were shorter mechanical reefers built before that of course.
Many railroads began dismantling their icing plat forms in around 1970/71/72 time frame. By then mechanical reefers had largely replaced most of the ice reefers in service. Some ice reefers remained in service through out the 1970's were in limited non-produce service from what I read, perhaps some meat packing service. IIRC, for produce (lettuce etc.) by the early 1970's, that was taken over by mechanical reefers and ice reefer service done and over with by early 1970's.
