Lionel track is available in three basic types. 0-27, O and Fastrack. Fastrack has gray roadbed attached to the track, which makes it perfect for temporary layouts or around the Christmas tree.
The basic difference between O and 0-27 track is the rail height. As a basic rule most any O or 0-27 train will run on any track, The exception being the large scale locomotives. Those require O gauge track and a large radius.
If you are not sure what type of track you have look at the metal tie that holds the three rails together. A brown color indicates that it is 0-27, if it is black in color it is O Gauge. If you are not sure what radius curve you require simply assemble half of a circle, measure it at it's furthest points (on the outside rail). If it is 27 inches across you require 027 curve, if it is 36 inches you require 036 curve ETC.