NMRA S-9 requires that at full load you must be able to deliver 12 volts to the motor.
Most DC power packs do not have voltage regulation. To deliver 12 volts under full load requires an unloaded voltage of between 16-18 volts. The reason is that as you increase the load the voltage will drop.
For DCC you need to add 2 volts to the track voltage because of the decoder and because of the transitions in the DCC signal (actually 1.5-1.8 is enough but the standards call for 2 volts.)
Most high end systems and many mid range systems have some form of voltage regulation but most still have some voltage sag under load. In addition with both DC and DCC there will be voltage losses in the wiring and track between the voltage source and the motor.
14-16 volts DCC is norm for regulated systems and 18 volts is needed for non regulated systems in order to supply the 12 volts to the motor under full load.
The other factor in voltage is speed. More volts translates to more max speed. Frankly most models have a top end speed that greatly exceeds the prototype. This can be especially true in some N scale models. To compensate for this it is common for the modeler to lower the track voltage in both DC and DCC by 2-3 volts.
Hope that helps
Stan Ames
