Your train must run on track, like the real thing. There is a vast amount of things readily available, and I suggest you consider your choices very carefully. Track comes generally with three different rail head materials- Nickel Silver, Brass or Steel. NS is definitely the way to go if you can, and it also the most widely available. The main difference is that it will stay cleaner for a lot longer time- it won't oxidize quite as rapidly. Brass is a decent alternative, and steel is the worst choice if you're looking for consistent operation and cleanliness. Steel looks the most realistic, though. Stay away from steel and brass if you can, as they are being phased out from the commercial market. Buy the necessary components to assemble the track plan you like.
Likely, while you're searching for track, you'll encounter code on the packages. Don't be alarmed- the number simply refers to the rail height, in thousandths of an inch. Code 100 is mostly used (HO scale) and is also the most widely available. Modelers use lighter rail (CODE 83, for example) if they want a more realistic look. The ties are often finer, amongst other details. Also, lighter rail is often used when modeling sidings.
Laying tracks correctly is the most important step, since running trains is the main aspect of the hobby. Trains just won't run on less than adequate rail surfaces. Be sure to read this section carefully!
There are many different types of track available. From straight sections to rerailers, here's what you'll need to know:
Turnouts- curved, straight? #4 or snap-switch? Powered or Insul-frog?
Available curved or straight, in many different sizes. The curved ones are measured by the radii of the two ends, while the straight ones go by a number, usually. The lower that number, the tighter the turn off the track. A SNAP-SWITCH is the tighest of all, in that it's made to fit into an 18" radius curve for HO. A good rule of thumb is to go (scale speed wise) double the number of the turnout. Go about 8 MPH through a #4 turnout. Also available as powered and unpowered units, or as insulated frog or power-routing frog. The insulated is preferred for the novice, because it is the simplest to use. With power routing, the electricity is sent which ever way the points are thrown.
To install track, the process is similar to that for roadbed:
*Also- make sure to include at least one terminal rerailer section of track in the plan- this will be needed later in wiring.
You may find it necessary to cut throught the tracks, at one point or another. It is handy if you use a hack saw, moto-tool and cut off wheel, or special rail-nippers.
Ballast refers to the rocks and stones placed between the ties to allow for drainage on prototype railroads. You can mimic these stones on your model using crushed rocks, available at hobby shops. Or, you can collet your own outside- just make sure it's pure, clean, and dry. Run a magnet through it to remove any metallic particles. Then, scoop it down, between the rails, and on the sides. Avoid getting it into switch machines by covering them up with tape. Use a small brush to keep the stones groomed and out of the way of the wheels in between flangeways and switch points.
Glue it down by misting it with water first. Mix water, white glue or matte medium (an artist's product) 50:50, and add a drop or two of dish detergent (to break the surface tension of the water and allow it to flow into the crevices). Apply this with an eyedropper.