Use the rolling stock fly out menu to place whatever locomotives and cars on the tracks you want, where you want. You can then travel to other parts of the map to add carriages to sidings, or other trains to the mainline that will meet or pass your train. When you make up a consist or two that you like, be sure to save them as named consists, so that you can use them later without building them.
You can use the rules menu to add commands to the drivers of each train as you wish.