It is the details which draw the viewer into the scene which you have created. There are a number of detail available such as automobiles, scale sized figures, miniature scenes (as above) and electrical accessories which help give you an enjoyable model railroad. It is possible to do too much of any thing, but when the little add-on details are properly placed, you will find that people notice such things.
The predominant figure manufacturer is Preiser, although Faller and Noch also manufacture Z-Scale figures. Another firm, Merten, is owned by Preiser. There are a variety of model trucks and autos available also, and your dealer can advise you as to what is currently on hand.