Pat Brown wrote:
I have some Railway Sleeper in the garden as a border. These were
put a couple of years ago and have now started to fade.

That reads like it's a problem.

I have been
told that I can't paint them as the oil in the wood would react
badly.

oh I see. I thought one of their most endearing features was that they
quickly fade to give a lived-in look and feel.

An oil based paint perhaps, but you should check the impact on a spare
sleeper. I imagine that anything you paint on a sleeper will fade and/or
chip quite soon. The result being worse.

I would like to return them to a darker natural wood shade
without risking any harm to the garden/wildlife. Can anyone suggest
the best way to go.

Replace them with 'new' ones I suppose.

Thought: Once again, I caution you should test on a spare, but a decent
application of linseed oil may bring out the effect you're after and,
obviously, will not chip, but it will fade.
--
William Tasso