The main cycling corridor in Hobart runs parallel to the rail line. The easement has been wide enough for over 11km to do this. Rather than a rails to trails, it's a rails and trails arrangement. It's one of the better off-road bicycle transport facilities in Australia, in my opinion.
To do it on a rail line itself, you'd probably need a *very* durable MTB with dual suspension. Then of course, you could do the Nullarbor Plain -- there is, I understand, a gravel service road that runs parallel to the line right across. It is a (fatal) attraction for Japanese riders of both bicycles and motorcycles. Most sane peope stick to the Eyre Highway which is further south.
And I suppose the key word in answering your question is "sane".