If you want the track between two nodes to be perfectly straight, you have to line up the control points with the nodes, but it's not easy when you have to zoom in. This tutorial shows you how to use a second track to act as a straightening guide:
http://simtrackipedia.wikidot.com/btb:perfect-straights
http://simtrackipedia.wikidot.com/btb:perfect-straights