I suppose that if copper ever became impossibly expensive, that an aluminum-clad steel contact wire could be used for electrified railroads. The contact surface would have to be steel with aluminum cladding around the rest of the wire. Of course, the steel contact surface would rust, and this would be rubbed off with use.

A very similar thing is used on the B.A.R.T. system. The third rail is steel with a steel contact surface for good wearability. Aluminum bars are bonded in the web of the rail for low resistance. This allows a smaller cross section (and lighter weight) of steel rail to be used.