Highest rr pass between the Appalachians and the Rockies would be Paisano Pass, west of Alpine, Texas, at slightly more than 5000 feet elevation.
Originally part of the Missouri, Arkansas & Southern Railway the tunnel was constructed 19 feet high and 14 feet wide. In 1967, the tunnel was expanded to 24 feet high and 19 feet wide.
My grandfather, Robert Pense of Winslow , Arkanasas, helped build the Winslow Tunnel. He was renowed in his community for his skill with an ax. My dad, Carl Pense, started jumping on the train in the tunnel at the age of about 12, riding to another community and buying a horse and small herd of cattle and taking them back to Winslow to sell. He moved to Illinois as a young man and became a successful farmer and livestock dealer, in spite of the fact that he never learned to read or write. Dad was killed in a truck accident in 1989 at the age of 72.