Hi Dave, yup you'll want to join those individual surfaces together (select them and run Edit > Join) so that they have a shared joined edge between them instead of how they are currently set up as just totally separate objects.

Once you have joined them, you can then select the front planar surface and fillet its edges with a radius of up to about 0.1 units.


> I've also tried using a blend surface after trimming round the
> plane with a rail surface, but can't make that work either.

If you have that version handy could you please post the 3DM file for it as well?

One thing with blend is that you need to select 2 surface edges to make the blend happen. If you have some curve construction geometry still hanging around it can pretty easily happen that the construction curves are sitting right over top of the surface edges and getting in the way of selecting the edges. If that's the case you want to delete or hide the original construction curves so that you can get them out of the way to make it easier to select edges.

- Michael