Careful, on Monday (Pasquetta) trains, planes and highways will be extremely crowded: this is a typical "travel" day when people will be coming home from a long weekend. I remember a few years ago when I needed 5-6 hours for Napoli-Rome on the autostrada! On the other hand, a Freccia in the morning with arrival in Rome in the early afternoon would be no problem. And there will always be taxis at Termini.