The voltage rating of the cables will be based on what voltage they're intended to carry.
Not really, at least not if railway practice is like everywhere else.
Most cables, at least for LV work, have a pair of voltage ratings, one for the maximum voltage between a conductor and ground and one for the maximum voltage between two conductors. Some cable types are available in variants with different voltage ratings.
However not all types of cable are available in all voltage ratings. In my experiance SWA is always 600V/1000V. T&E seems to be 300/500V. Even with cable types like flex where multiple voltage ratings are available, robustness concerns may drive the choice of a higher rated variant.
So the voltage ratings of cables often far exceeds the operating voltage of the circuits those cables are used for.