Just found this recently on Open Railway Map. It shows that I-20 from Shreveport west to Greenwood was built on an old RR grade. If you follow the link, you will have to navigate to the area.
https://www.openrailwaymap.org/ (https://www.openrailwaymap.org/)
Good find. Looking at it, appears to be an old parallel route to the current UP route that still exists. MoPac probably had some money waved in front of them on what amounted to be an unused ROW and the rest is history.