Thanks for all the replies and welcomes!
I've decided just to use a 'template' from the internet which follows very similar principles to the 'chords' method of determining the curvature of railroads...
I've opened it up in Illustrator and below are the results...
It looks a lot better than my original guess, but again, this isn't even going to happen, I just need to show that I have looked into it and apply some from of 'rule' to the railroad design.
A bit of a vague question, but does this curve look 'right' to you? (it's the orange line, the black wheel is the template I've found)
Thank you
https://dl-web.dropbox.com/get/curve.png?w=03422b2c