As Andy says, gradients are a problem when doing this. It's fine placing the spline point over the track splines and matching the heights but in between, there can be some wild variations.
There is a catenary height tool but this is no good for UK wire height. I've searched the DLS but can't find any alternative. If anyone knows of a UK height tool, I'd be grateful if they would let me know where it can be found.
Cheers.