Unless you are exaggerating the shortness of the time, this overpass had to have already been in the pipeline. You can't get a design done and bid and any work even started in much less than a year even if it is a high up the chain politician's priority.
Tennessee has a set of criteria to prioritize crossing signals and grade separations, that include traffic volumes both road and rail and speeds both road and rail. However, in case of signals and gates, a fatality puts a road crossing at the head of the list, and if not number one only because there is one already there under way, or if there are those in the same time frame with multiple fatalities. Therefore, in the case mentioned, this road crossing would go to the head of the list regardless of any real or imagined relation of the fatalities to politicians. However, if the time was only a couple weeks, it again suggest that this work was already in the pipeline unless the materials were mooched off those intended for another crossing already designed and materials ordered. By the way, southeast of Nashville suggests the main to Chattanooga & Atlanta ex L&N ex NC&StL line which is heavily trafficked, and least in the first 30 or so miles out of Nashville relatively straight and fast.
