Gavin
Matt tells me that building models where the Track Centerline is your starting point is possible, but it is harder work than building the models of the Rail Strings if you can get them. The recommendation is ask the designer to provide you with the Rail Strings for each Track that you are looking at and those are the best and easiest starting point for the model.
Matt tells me that the Twist of the Track is easy to measure and validate with the GEDO system and he would be more than happy to talk through this with you. He tells me that it is the error between the Track CANT and the Design CANT as measured over 3m (the distance between the bogies of the train carriages - if it gets beyond the allowable tolerances (which are based off a standard that uses the curvature of the track and the speed design for the track) then it is cause for derailment of trains. The GEDO system was designed to tackle this specific problem and Matt is very familiar with the process involved.
If you want to send me your contact details (alan_sharp@rockpilesolutions.com) I would be more than happy to connect you to Matt as a follow up.
Alan