Hi Tom, it would probably help if you could post the 3DM model file as well.

Do you want the gem object to deform at all or do you want it to only be moved and rotated? If you don't want any deformation I'm not sure that there is actually any possible solution to the problem as stated, if the rails are not planar and have different lengths and different tightness of bending in them.

If you want them to be deformed you could try building a surface between the rails using Loft or Sweep and then use Transform > Deform > Flow.

- Michael