There are multiple factors that affect the behavior of transmission lines, including their top transmission frequency. To start of simple, the gauge of the wires will determine the maximum current rating, and the insulation between the wires will determine the maximum voltage rating.
From here, the next topic to visit is Telegrapher's Equations. These equations break down how in every transmission line there is line resistance and inductance along the wires, and conductance and capacitance between the wire pair.
In lossless transmission lines, the transmission speed is infinite. However, lossless transmission lines only exist in theory, as all conductors will present some resistance and all insulators will present some conductance.
The problem is these lossless lines will degrade the signals the further they travel. In the case of sending a square pulse for example, you might remember a square pulse can be simulated out of multiple sin waves through the Fourier Series, each with a multiple frequency.
Each of these sin waves will travel at a different speed down the transmission line, as the lossy transmission line model is dependent on frequency. Thus, as the wave travels down the line, the pulse will slowly become deformed, first around the corner edges, and then until it is no longer recognizable. I tried finding an image of this concept but was unsuccessful, invite anyone else to post a link
Thus the transmission speed is limited by the parameters of the lossy transmission line. The line works up until the individual pulses are no longer distinguishable as 1's or 0's at the other end.