I'm putting six 022 switches in a post war o gauge layout and want to supply fixed voltage to power them.  I'm using a KW transformer and there are two choices for the fixed voltage.  Terminal C supplies only 6 volts which is insufficient to operate the switches.  The other choice is terminal D that supplies 20 volts.  The instruction manual for installing the switches says to use terminal D but has an * note by it that says "If voltage obtained from D post of KW transformer is too high, add an adjustable 10-ohm , 25-watt resistor.
When supplied from terminal D, the switches operate fine and the lights are bright.  My question is:  How will I know if "the voltage from the D post is too high"?  I have measured it to be 19 volts with the 6 switches installed.  I don't want to damage the switches with too high voltage.  Thanks in advance for any help or advice. 

Thank you for the advice and link to the source for the bulbs, but I need a solution before arrival of the grandkids next week.  Today I moved the leads for common or ground from U to C on the KW.  That resulted in 14 volts instead of 20 at terminal D for the fixed supply to the switches.  They operated fine and the bulbs were not nearly as bright as when the 20 (19) volts were being supplied.  However, the problem with this setup was that the variable voltage to the two trains was too low to operate them normally.
I thought about using another transformer for the switches, but could not decide where to hook the common or ground lead of the second transformer.  Would I connect it to the U terminal on the KW?
Your other suggestion to operate the switches from one of the two variable supplies might also be the answer.  Instead of operating two trains on separate loops, I can put them both on one variable supply to operate one train at a time and use the other side for the switches.  If I can't use another transformer, this will probably be the best solution for now.
Thanks again.