Put another way:
The rating of the coil is important in most circuits. Also AC VC DC rating of the coil is a factor to consider- it's all about overheating the coil with extended ON time. AC being alternating and thus having the zero crossing means off time the coil is not powered, where with DC current the power is always flowing thus more heat. Also, lower voltage may activate a relay coil but at some point there is slow activation or not enough power to activate reliably. So ideally there is a minimal range of voltage they should operate. The upper range of voltage is that again, more voltage is more current, and DC VS AC play into a factor at what point a given voltage along with ON time of the coil determines a practical limit.
The switch contact rating- the voltage indicates the safe maximum voltage the contacts are rated to turn on and off. Higher voltage could arc across the contacts. So effectively this is more of a maximum value than a recommended operating voltage. Again, you can run much lower voltage through a contact it's just 2 pieces of metal touching when we get down to it. Lower voltage at some point on a high voltage rated switch contact may not be self cleaning, but again, for model train discussion, nothing wrong with using 120-250V rated contacts for our typical low voltage (8-20VAC) AC functions on a layout. DC is slightly less desirable but again for the most part in the model train world, we are switching below 25VDC and the contacts are not likely to arc.
