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In networks capacity_of_link = bandwidth*propagation_delay
My doubt is how can a same wire carry multiple signals? Is is because the wire is very long (still the wire can be either high or low). I can understand if parallel wires are the reason for increase in bandwidth and thus capacity increase.
the wire can be either high or low
This assumption is wrong and is the source of your doubt. At high speed it's perfectly possible for one end to be high and the other low. A transition ("edge") from high/low takes time to propagate along a wire. It doesn't propagate at the speed of light either, but at a rate determined by the capacitance of the cable.
At high ethernet speeds it's perfectly possible to have multiple bits in flight along a cable.
Ethernet standards use signal modulation to increase bandwidth without much frequency increase.
For instance 1000Base-T use PAM-5 modulation, it allows to transmit 2 bits at a time:
And for 10GBase-T a PAM-16 modulation is used, allowing around 4 bits to be transmitted at the same time.
