Converting PC Power Supplies

[Skullptor]

I am converting a pc power supply to be used with model trains to power low voltage items. The two PS I have do not have a white wire which is typically -5V. Is there another wire that can be used if the white wire is missing which will produce the -5V?

 

[clutchc]

Here are the pinouts for ATX power supplys: http://cdn.overclock.net/2/2b/500x1000px-LL-2bf838c0_st45sf-g-modular-pinout.png
Be sure to take note of the capacity of the 5V rail(s) as printed on the nameplate.
While only +5V is listed, reversing polarity will give you what you need.

This may be easier to read: http://www.helpwithpcs.com/hardware/img/power-supply/atx-psu-pinouts.gif