No, the problem is that Interstate 10 is more of a straight shot between New Orleans and San Antonio. Also, the train makes way more stops than you have to you in your car *fuel and food notwithstanding*.
The train takes a route that us Louisianians call "the back way" through Schreiver, New Iberia, Lafayette *my stop*, Lake Charles, Beaumont, and then into Houston, and on to San Antonio from there.
On I-10, you can go from New Orleans to Baton Rouge in 2 hours, Lafayette is 1 hour west of Baton Rouge, then Lake Charles is 1 hour west of Lafayette, then you can bypass Beaumont without a glance lol. Houston, well you cant miss Houston, and then on to San Antonio from there.
Also, on the interestate, you dont have to deal with freight interferance from BNSF and UP along the way, that can really delay the SL as it did for my trip Friday, whereas the Interstate normally flows pretty easy.
It really comes down to time. if you have the time to relax and enjoy the ride, then take Amtrak, if you dont, then your best bet is to drive to San Antonio from New Orleans.
Helpful tip, if you are going to drive from New Orleans to San Antonio, avoid driving through Baton Rouge around 4pm, the Interstate gets gridlocked between the 10/12 split, and the Mississippi River bridge.
Just a local's 10 cents on the issue. If you have more questions, feel free to ask.