Hi Z7, I am not sure if there are different versions of the water rail, but on mine I originally didn't have a header tank so filled through the thermostat housing to just below the overflow outlet. I remember speaking to Raceline at the time who suggested this was the way to go. I subsequently fabricated my own header tank that feeds into the thermostat housing overflow connector (and replaced the thermostat housing pressure cap for a blank one). Despite now being theoretically able to fill through the header tank I still find myself filling and checking through the thermostat housing cap. The header tank essentially just preventing a the loss of coolant into the overflow bottle.
