Thank you for responding. I purchased the GGD 3-rail Illinois Central RPO and baggage cars from Beth at Public Delivery Track. They are very impressive models and are currently used as add-ons in both my MTH "City of New Orleans" and the "Panama Limited" passenger trains.
My current goal is to create a regional IC train with older passenger cars (Madison vs. smooth side) hauling mail and passengers. Though I and pretty sure that outside of finding some customized painted cars, there is little chance of finding the orange and chocolate scheme in 3-rail.