He would have made the troops more mobile using trains and created a whole new paradigm. As such he was only allowed to do it a few times and it worked spectacularly.
All the Confederate commanders used trains when and where they could. Johnston used them to shift troops during Sherman's drive on Meridian. Lee used them to get Jackson from the valley to the site of the Seven Days battle, and used them later to send Longstreet off to help Bragg at Chickamauga. If the Confederates didn't use rails more to shift troops then it's because they either needed them where they were or the rail system post 1862 or 1863 had deteriorated to the point where the trains weren't available.
Don't get me wrong, I am not a critic of Longstreet and I firmly believe that he was the best of Lee's subordinate commanders. But to but him in overall command over a Lee or a Grant isn't justified in my opinion.