Common Mode Vs Single Ended Signals
I think you're mixing things up !
A common mode signal is "average" voltage of a differential signal. A differential signal is a signal across two conductors
where when one line goes up a certain amount of voltage, the other goes down.
In formulas:
Vcmm = Common mode signal, for example 0 V or 1 V DC (it is a constant voltage)
Vsignal = the signal we want to transport, for example a 1 kHz sinewave
with an amplitude of 0.5 V

signal on positive conductor:
Vdiff_pos = Vcmm + (Vsignal / 2)

signal on negative conductor: Vdiff_neg = Vcmm - (Vsignal / 2)

So you can derive from this: Vcmm = (Vdiff_pos + Vdiff_neg) / 2

And Vsignal = Vdiff_pos - Vdiff_neg

A single ended signal is a signal referred to ground.
In a differential signal only the common mode voltage relates to ground.

If the ground voltage changes then a single ended signal will change because
it it referenced from ground.
If the ground voltage changes then a differential signal will remain the same
as it will be the common mode component that changes. But since the signal is
Vsignal = Vdiff_pos - Vdiff_neg the Vcmm is eliminated !!!
This is the main reason why differential signals are used. All disturbances
end up as a common mode and can be eliminated by subtracting the signals.

A way to transport signals is through transmission lines. Single ended transmission lines have a ground. It is however possible to make a differential transmission line with two conductors and use that for differential signals.
But transmission lines have nothing to do with how you represent a signal, if it's differential or single ended I mean.