Since most--or all--of my track will be Eishindo flex-track, I experimented with a few different techniques to get a decent appearing, reasonably fast-and-easy to install system. After a few experiments, I landed on a system using uncured butyl rubber (AMI Instant Roadbed). My roll of old AMI roadbed is light gray, 1/8" thick (about 3mm) and comes in a 30-foot roll. It is a semi-soft material that flexes easily, can be painted, and (importantly) retains shapes pressed into it. AMI went out of business, but you may find some old rolls at your LHS or, you can buy butyl rubber rolls from auto parts stores where it's sold as air conditioner sealer, or roofing patch. You will probably need to paint it because most of what I've seen is black, but that's ok. You can even press T-gauge ballast into it if you wish. Below is my technique for your consideration:
First, I cut the rubber lengthwise into strips about 12mm wide using the x-acto knife. Next, I cut these strips again lengthwise in half, but I bevel the slice about 45 degrees. Once you peel them apart, This way you end up with two "halves" of the ballast, which (similar to cork roadbed) can be butted up against each other with the beveled edges out representing the slope of the ballast.
Using your center line as a guide you can then install the ballast on the layout.
The next step is the track itself. I use the gray tie flextrack. The gray flextrack actually looks like old faded ties or concrete ones. I paint the whole system (rail and ties) roof brown. You could also use rail brown. Previously, the track will have been bent to shape, and once dry you just press it into the soft ballast roadbed. I use my thumb but a small block of wood can be used to try ensure the rail is level.
The track system at this point looks pretty good, and you may like it just as it is. (You would still want to spray the whole thing with Dullcote to keep dust from sticking to the balast.) In my case, it didn't look quite good enough, and I wanted to finish it off a little more to add realism. This is again pretty fast and easy: First I give the ballast a wash of light gray. This blends in some of the flecks of brown in the material. Next, using a medium grit sanding board, I press the shoulders of the ballast to a more prototypical profile, I try to eliminate any large bumps which would not exist in the real world. It also imparts a very fine granular texture. A prototype photo is always a good thing to consult when doing this.
The final finishing steps are to dust the whole system (using a photo as reference) with a little white or light gray weathering chalk. This added more white highlights which I wanted. If you were modeling an area with a flange greaser or engine facility, you can add black, or an area that sees a lot of sanding even more white or tan-- whatever you deem fit. Then, very importantly, seal the whole thing with a good coat of Dullcote (flat lacquer). The rail heads will need a good polishing with fine sandpaper. That's it!