Delivering by train is, overall, a sound strategy. The break point from an economic perspective on truck v. rail is usually about 750 miles, and about 1,000 miles rail reigns supreme. Trucking cars in open trailers has probably increased the number of car damaged in transit -- a figure Tesla obviously wants to minimize.
There are, however, obvious downsides to rail:
1. Transit times can be long and inconsistent. The rail route from the Union Pacific-served Fremont factory to CSX's Automotive Distribution Center in Birmingham, AL is roughly 3,000 miles. Interchange locations between CSX and UP include, among others, Chicago and Memphis, but neither route is very direct. New Orleans is another possible spot.
2. Automotive trains are not particularly high priority on most railroads. Passenger traffic (if applicable), followed by premium intermodal, and then everybody else is usually how it goes. Regardless, trains are rarely on a tight schedule.
3. Contamination with metal dust. Even in the special railcars used to transport automobiles, fine particulate metals can hit the cars (usually metal shavings from wheels, etc.). The detailer doing the prep work at the Tesla Service Center may not realize this and end up swirling the paint -- something Tesla seems to do anyway.
For those who are interested here are what the routes look like:
Finally, here's a satellite image of the CSX distribution center.