When current flows down a wire, there are 4 properties that can come into play.
1) Wire Resistance
2) Wire Inductance
3) Wire Magnetic Fields (Magnetic Coupling)
4) Wire Electrical Fields (Capacitive Coupling)
Which properties are OBSERVABLE or measurable depends on the TYPE of current running down the wire. In other words it depends if the current is pure DC or has any form of AC which includes pulsed DC.
When running PURE DC current down the wire, THE ONLY PROPERTY that is observable is #1 in the form of voltage drop down the length of the wire. In other words the only thing we needed to worry about was the resistance of the wire.
When running any form of AC current down the wire, then properties #2, #3 and #4 can become observable. The DEGREE that they become observable DEPEND ON BOTH the amplitude and frequency content of the AC current.
It is these properties, that were always there, can sometimes get people building large layouts with DCC potentially see strange problems appear and at a loss to explain what, why and what to do about it.