My (highly limited) engineering intuition says: Dual railing is good - for balance, for switching of tracks, for lightening load and making sure it is applied in basically the same direction always, and perhaps other reasons I haven't thought of.

So, why ever have monorails? I mean, ok, if you want to fit into some super-narrow crevice then maybe I can see the motivation, but other than that - what are the benefits?

Note: I'm interested in non-magnetic monorails, although if you want to provide an answer for maglev's as an interesting side-note that's also fine.