Two plus years later, I'll revive the old thread and hope the OP still cares.
The "R" had nothing to do with assignment to work train service per se. At some date which I don't have the feds mandated that various pieces of rolling stock which did not comply with all the regs had to be designated as "Restricted". This was shown by the letter R after the number, and was followed by a word or two which stated the reason. Among the reasons often was "AGE" for cars beyond the normal retirement age. Other reasons might be "DRAFT GEAR" or some other obsolete component.
The above I'm sure of. Now the following I'm searching my memory and may not have 100%. I THINK the restriction applied to interchange. So a car beyond X years old (40? 50?) was still allowed to be used on the owning railroad but was not able to be interchanged with other lines. And it's quite possible as mentioned above that there was a speed restriction in some cases but I can't say so for sure.
Also, I believe this requirement is no longer in place having been modified to accomplish the same results but through the equipment database, but I'm not positive on that.
But while MW service was an obvious place for such equipment to live on, it was not the only assignment. I recall seeing former PRR cabooses in blue paint with an R after the number, in local freight service.