Old railroad tracks have a variety of uses, ranging from sections that are cut down to be used as anvils in forges to decoration for backyards that have replica trains. As thick pieces of metal, there are only a few ways you can reliably cut through them, and all of the various methods have a basis in heavy construction. As long as you have some spare time and the willingness to learn, you can use a variety of tools to cut a track down to size.
One of the simplest tools you can use to cut large pieces of metal like railroad tracks is an angle grinder. While the 4-inch variety is too small to handle the size of a piece of track, the larger 8- and 12-inch versions have more than enough horsepower and size to handle the job. A metal blade is required, as is the physical strength to wield the machine, but it is one of the best ways to accomplish the task.
Miter saws are more commonly associated with wood jobs, and are known as chop saws because they are lowered into the wood to "chop" off the end of the cut. However, you can also insert metal cutting blades in place of wood ones, allowing you to cut pieces of metal. You will need a larger version with an 8-inch blade or bigger, but the same principle applies. The track is laid in the tray of the saw and the blade pulled down into the metal to cut through it.
Although it might be difficult getting your hands on one given the fact they are usually only available to professionals working in the field, there are specific rail cutting saws that are used in the railway industry. They are the same design as the larger angle grinders, with a motor and blade body that is held in both hands and pushed down into the track as you cut. If you have access to one, this is your best option for cutting rail tracks.
While it does require a bit of training to use one properly, a cutting torch is one of the easiest ways to cut large sections of metal. They work better with sheet metal than with large sections of steel or other material due to the fact that they melt the metal to cut it rather than actually cutting through the material, but they are still a viable option. If you have one in your shop or have a friend who has one, this is another option for cutting tracks down to size.