Thin coat of light oil...

Put a thin coat of a light oil across the tops of all your tracks, spray the whole thing black, then wipe off the tops of all the tracks. Just be sure to also protect your switches as needed. 

Randy

Concur

Agree with Randy

OR...

You could use Pinstripe Tape to cover only the rails.

Tape = Yes, Oil = Definitely NO

Dear Ken,

I've used 3M painter masking tape (blue, lo/mid tack) to mask just-the-rails for spraycan bombing sessions previously, Tamiya RC model "stripe" tape is another good suggestion (although possibly dearer that 3M tape... ).

http://www.hobby-time.com/index.php?main_page=product_info&cPath=16_681&products_id=54024&zenid=t45p66mbq8hg1g33j226188ch0

(The narrow 1/4" width allows you to accurately tape just the railheads and inside railhead face, while keeping the track-centres clear for painting).

I would _strenuously_avoid_ putting Oil or other Wet substance (before someone mentions WD40) 
on the rails for _any_ reason,
the seconds you (might) save now are the dust-adhesion/acccumulation, wheel/rail crudy-ing,
and multiple hours-of track-cleaning frustration you're setting yourself up for later...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

1/8" or 1/4" Pinstripe

you should be able to get 40 to 80 feet of 1/8" pinstripe from your local AutoZone type store for about $10

or on ebay for less: (1/8, 1/4) pinstripe -marine

Tape and airbrush

Blue mask tape across the track, paint the benchwork black.

Then use a airbrush and paint the track with flat black ( or rail brown ) paint, clean off rails. Done.

Ive used the craft paint with good success in the airbrush on track. The nice thing is once its dried, it comes off with a few wipes with the rough side of a masonite block.

I painted the space in my staging yard with the same rail brown and it helped get rid of the open look, covered up the blue foam and helped tone down the area.

Why mask?

I would think it would be easier just to spray on the paint then wipe off the rails. Masking switches is probably a good idea, but a rag or paper towel with a little solvent suitable for the paint being used should take care of the rail tops. If the paint is water based, then that would be water as the solvent.

Plan B

How about brushing it? Drop a couple of lengths of plastic channel over the rails where you are working and have at it. Shouldn't take any longer than masking and/or cleaning all that rail.

pqe

Paint it all

Spray the whole thing, wipe the rail heads. A tight weave rag damped with lacquer thinner pulled tightly on a finger tip works great for first pass mass removal. Wipe in a single stroke from one end to the other, pick a new spot on the rag, wipe the opposite direction. Careful when you come to switch points.

For the final step a small block of wood ≈2"x3" wrapped with thin tee shirt material so there is a single layer of cloth on the face. Wad up the remaining cloth on the back of the block. Pull very tightly on the cloth on all four sides into the wad on the back. Hold the wad tightly so the cloth on the face stays taunt across the face. Dampen face with lacquer thinner. Slide along both rails occasionally repositioning block so as to present a clean path across the cloth.

This two step process has been working excellent for me. Rail heads pass a white glove test afterward.