I think you are underestimating the amount of engineering involved with connecting the rail line directly to the terminal. There are a lot of limiting factors to take into consideration. The station would have to be at the end of a wye, thus requiring curved approaches from both directions. The line would have to weave its way into the complicated network of access roads already serving the airport as well. The wye would have to start/finish at a point no farther north than the south side of I-40, because a bridge or viaduct over the roadways north of that point would penetrate the clear areas required at the ends of the runways. Because of that, the curves for the wye would be a tighter radius. I think given the traffic volume at RDU, the expense of such a setup would be prohibitive from the start.