It's interesting to consider the existence of forward and backward currents in transmission lines in which the VSWR is greater than unity. That is, a Tx line terminated in something other than its characteristic impedance.

How do I measure these line currents in practice?

Presumably using something like a directional coupler from which to infer the forward and backward currents based on the coupled readings.

One typically places the directional coupler between the RF source and the Tx line input. So the directional coupler is "measuring" the currents external to the transmission line - not the actual forward and reverse currents in the line itself.

One can readily propose a situation in which we discard the Tx line altogether and replace it with a passive discrete network having the equivalent line input impedance. We will still measure the same "forward" and "backward" signals via the directional coupler when in fact there are no travelling waves involved.

 

It is a fact of wave theory (and experimental observation) that when two waves travelling in opposite directions meet, they interpenetrate, pass through each other, and continue on their separate ways. This can be seen visually in a hydraulics tank with water waves and is a textbook experiment.

So if two observers A and be were stationed at each end of a transmission line and they send each other a signal together, each will receive the other's signal.

So much for a simple single signal.

The difficulty comes if they are sending continuous signals because each then has to distinguish between his own signal and the received one.