AC & DC Circuit

In certain applications. not for general power, but High frequency they can. But 2 separate systems each fed with 2 conductors, can share one, but these are no common practices. Each circuit should have its dedicated lines or cable. When you use only the cable and the systems separately and not at the same time - the cable will conduct AC or DC. Both together doesn't work because the nature of the power is different. Both circuits will oppose to each other.

They can.

• In railway signalling applications in AC traction areas, one of the running rails is used as the return path for AC traction current and for DC track-circuits for the signalling. The converse is true in DC traction areas. It can be said, therefore, that one of the cables, i.e. the 'common return', is sharing AC and DC circuits.
• In radio antenna operations where masthead rotation equipment is used, it is not unusual for the coaxial downlead conductors carrying the HF, VHF or UHF signal to carry also the DC power that drives the masthead rotator.

The secret is to separate the two signals correctly at each end of the cable.

In some US systems we use both rails for dc propulsion current and put an ac signal on both as well. We use center tapped transformers (impedance bonds) to define the track circuits and couple the signals.

Superimpose AC over DC.

The losses - additive (RMS AC, DC).

Insulation (peak AC, DC) .

All other line components - based on either of the above (depending on characteristics of them).

In the feeder end, you need a transformer to merge the two.

In the end you have to put a filter to separate them out.

What one gains?

Except in the transmission carrier signalling - where the high frequency carrier is used to communicate, especially the faults/ other information through transmission line, where the signal is negligible with respect to power and hence do not affect any of the above.