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