Most of ATL-BHM on the ex-Southern, 166 miles, is one curve after another. The joke is that the surveyor of the Georgia Pacific was paid by the mile. Most of it is also a single-track line, albeit with frequent sidings, that carries a high number of freight trains. 4 hours is not unreasonable, given these factors. Even if you ran at maximum track speed with no interference from other trains, I suspect 3:30 is as fast as possible without spending hundreds of millions of dollars to straighten out the line.
The reality is, ATL-BHM can be driven on I-20 in 2.5 hours under ideal traffic conditions. The good news for the Crescent is that south of Birmingham and north of Atlanta, the ex-Southern is a much faster railroad.