This article is a couple years old but contains a blurb about RTP:
Why Companies are Moving Back Downtown
One place attempting such a reinvention is Research Triangle Park (RTP) in the Raleigh-Durham area. The campus is half the size of Manhattan, and boasts several global science and tech companies, notably IBM and Cisco Systems, but it’s also an artifact of 1950s community planning. The fact that tech companies like Red Hat are choosing downtown Raleigh over nearby research parks illustrates the problem RTP currently faces: It has no housing, no light rail, and no main street with cafes, restaurants and shops. The RTP’s layout inhibits the kind of informal socialization and networking between tech workers that is increasingly common in urban innovation districts.
This description is accurate in my experience. The company I work for had our office on the west edge of RTP just outside of RTP proper. IBM, Cisco, Ericsson, Qualcomm, and other electronics companies are our customers. As companies like IBM and Cisco scaled back their operations and Ericsson left completely, our customer based largely migrated out of RTP. Eight years ago we moved from RTP (technically Durham) to Cary where we are closer to the airport, customers and where most of our employees live.
When the northern half of 540 was first built. it would have been a good time to add light rail from north Raleigh and Wake Forest to the airport and RTP. It was all brand new development and wouldn't have caused a retrofit to add light rail like it would to build from north of Raleigh (e.g Wakefield) to downtown. To keep costs down, most of the light rail proposals for the Triangle have involved reusing existing rail lines which I believe are owned by Norfolk-Southern and also used by Amtrak neither of which apparently want to allow other traffic on their rails.