The only easy way is to walk along the railway line (although i know you are not supposed to!), anything else is to rough. Try to walk it between trains, although goods trains also can go through at any time inbetween. It only takes about 1 hour to walk from the road, you will find that you will also have to walk along the centre of the track for much of the time, as again its to rough on either side.
Coming from the east (F William) direction you come to a fairly long cutting (a few hundred meters) and you would not want to be there when a train passes!
The west side is slightly shorter and has no cuttings, and also has a good parking spot right by the track where you would start if you had a car.
I was in both in Oct, both are good well maintained bothys with fireplaces, but you'll have to search for fuel (no driftwood on the beach by Peanmeanach when I was there... I took in my own coal)