Are there any effects on capacitance and inductance when operating at audio-frequency (i.e. 1kHz - 20kHz)?
Skin effect is not a consideration at these frequencies, they barely start becoming an issue in the low MHz range

Capacitance will increase how much load the audio amplifier will have to drive as things get longer, while they are generally built to compensate more of an inductive load. so some resistive loading on the amplifier may help if it is struggling, or a smaller resistance e.g. 1 ohm in series with the output could improve things,

If the signal is across both tracks then you end up with similar to a bifilar winding, meaning a lot of the inductance is cancelled out, 

Various parts may act as antennas, but at the frequencies involved the energy that could couple in would be extremely weak

Main concern would just be the amount of capacitance the amplifier has to drive, and the amount of resistance, 1KM of even copper track assuming model railway would likely be tens of ohms if not nearing 3 digits.