I hope this helps;

If the rails are cable thru stanchion-
-then usually--though not always..
the cable is darker grey or even black

I use a black indelible pen such as a PENTEL ( not Sharpie--they bleed)
on an absorbent background, paper/ tissue

I then install the railing--ensuring --where applicable -
-that there is a stanchion on a corner etc
and cut and shut the rails to achieve this.

I then pick out the individual stanchions
in hull colour with a fine brush.
ther after LIGHTLY drybrush in a downward direction with a medium grey to visually narrow the impact on the eye of the rails

...==> works for me!

In greater detail and with supporting images to the above

read here:

http://www.shipmodels.info/mws_forum/viewtopic.php?f=2&t=314101&p=926936#p926936