We inherited a dado rail in our lounge and SWMBO likes the idea of
painting it in a colour to contrast the walls. However it is currently
stained a mid mahogany is this feasible ?

I wouldn't worry unduly about the stain. If it is a glossy varnish, you'll
need to rough it up with sandpaper before painting. As it is starting off
dark, I would first coat it with a suitable undercoat. I tend to use Glidden
Primecoat Acrylic Primer + Undercoat, although the polluting high VOC types
will insist you use one made from smelly chemicals instead. I would then
finish off by a very quick rubdown with extra-fine paper with an acrylic
eggshell/satin or acrylic gloss top coat.

Christian.