While I was a resident of the South (Georgia), I scrounged up some money and took a weekend trip to Chattanooga. We rode behind the 610. the 4501 was scheduled to be on a steam trip the previous weekend in our neck of the woods but the train arrived behind a diesel (repainted into Southern livery for an anniversary celebration). If memory serves, the answer I recieved at Chatanooga was the 4501 was sideklined for an overheated bearing, or bearing related issue. One beautiful locomotive. Southern Green is a stunning color in the sunshine. TVRM is an excellent place to visit, and hope to return again someday.