I thought I'd post an update, despite not solving the problem in rail clone. I have basically decided to do what Rokas suggested and make/edit the spline to work as I want it to. This is mostly because I discovered that it's not as difficult to get it correctly spaced as I thought it would be. Here is my solution (captions below the images):
The part circled in red, shows that the current railclone object does not work, with the bars conforming to the curve rather than the rails. The posts, indicated in Blue, as however in the right places, and there are the correct number of them in the area circled in green. Therefore I detached the part of the original spline circled in green.
Once the spline is detached, I applied a Normalize Spline modifier to it, which has had some significant updates to it since Max 2017, and is now rather useful!
Checking Show Knots is helpful, and Make Segments Linear. However, this does not make the vertices into corners, so when you re-attach it to the original spline, you still need to set them to corner.
Finally, when you count the posts, you need to make sure you add the start and end post to the count so that you get the number on your final Knot Count correct.
Select the original spline, attache the updated one, weld the vertices and set them to corner, and you should get the correct result.
I hope that is helpful to someone else.
If someone else finds an in-railclone solution I'd also be interested.