After you've placed and wired your model railroad track, but before you ballast it, a little paint can go a long way toward improving the appearance of its uniformly coloured, obviously plastic ties and shiny nickel-silver rails. With just a little effort, paint can create the look of ageing wooden ties and rusty, dirt-coated steel rails. Indoors, use water-soluble paints like inexpensive craft-shop acrylics, thinned as needed with water or (for faster drying) isopropyl alcohol. Outdoors, or with adequate ventilation inside, you can apply solvent-based paints, thinned with mineral spirits and brushed on or sprayed from an aerosol can.
Coat the top of your rails with a film of lightweight household oil, the kind used to lubricate electric motors, sewing machines or even hair clippers. The oil will add nothing to your paint job but it will ease cleanup later. Take special care to lubricate turnout components like the points and throwbar. You can protect these important areas by covering them with masking tape and painting them with a brush only.
Change the ties' appearance, from obvious plastic to something resembling oak or other hardwood. Real-world timber ties, more than 3,000 of them per mile of right-of-way, provide a track's base layer. Steel rails, laid at right angles across the 8½-foot ties, are spiked into place through the ties. Ballast, a coarse gravel, holds the ties in place. While ties start in multiple brown hues, most weather to varying shades of grey before being replaced.
Apply a coat of light grey paint using a brush, airbrush or, if your layout area is well-ventilated, an aerosol can of spray paint. Anything that will apply the paint will work. Neatness does not count here. Ballast, additional painting and scenery will ultimately cover most of this application anyway. But the grey paint will provide a primer coat and allow you later to suggest ageing ties. Let the paint thoroughly dry.
Pick a shade of brown that would later be appropriate as an earth colour along the right-of-way. Some modelers specify their preferred colour---a pricey Rail Brown, let's say---but any flat, earth tone brown in any cheap acrylic or solvent-based paint will do the job. Dilute the paint with water, alcohol or thinner, if necessary, just enough to achieve a smooth flow. Paint the ties completely, without worrying about misdirected splashes or uniformity of the colour. Indeed, the more uneven the brown tones---from dark to light and even barely visible---the better.
Enhance the ties' uneven look. With your paint tacky but not dry, rub it with a dilutant-dampened rag, thinning the brown enough to let some grey show through. The more visible the grey, the older the tie will appear. If you remove too much brown, reapply the earth colour or add other shades of brown. Or do both. To improve the ties' random appearance, clip a sliver off the end here and there with rail nippers, breaking their machined conformity.
Paint the sides of the rails with a thin brush and combination of the earth colour and an appropriate red. The colours represent both the splashing, wind-blown earth surrounding your tracks and the inevitable rust that attacks steel rails everywhere. In broadest terms, main lines are more brown than red, sidings and other lesser-used trackage more rust than brown. Adjust the red/brown mix to suit your tastes and track use.
Blend your paint job (after ballasting) with an overall coating of grime, a mixture of one part black paint to 20 or more parts dilutant. For best results, deliver the wash by airbrush or pump-bottle mist rather than brush, covering as evenly---and lightly---as possible. The objective in this step is not to "weather" the track but to pull together its various colours under a consistent tone.
Wipe clean the tops of the rails. Start with an absorbent cloth or paper towel to take up any remaining oil. After you've removed what you can with the cloth, follow up with an abrasive cleaning block, known generically as a brightboy. Make certain you've erased all traces of paint from the rail tops, which conduct electricity to the trains.
Save your good modelling brushes (and money) for your nicely detailed models. Brushes for painting the trackwork range from small to large, but their price tag should be consistently low. Pick up bargain brushes at any crafts store or press some over-the-hill veterans on your workbench back into service. An old toothbrush could probably handle the tie portion of this job.
Solvent-based paints must be applied with adequate ventilation. If you're in doubt, go with acrylics. Once set, usually overnight, acrylics can be as durable as an enamel.