Are the controllers and transformers powerful enough to drive more than one train at the same time? From experimentation, it seems they certainly are.
With just a simple loop you can run two trains on the same track and one just follows the other. Unfortunately, one will usually be faster than the other and will eventually catch up and attach itself. So this is not very exciting after a while.
With a passing loop you can run two trains on the same layout, and use the passing loop to stop one train while the other one goes past. If you're quick enough, you can use this method to hold up a slow train while the faster one overtakes it, after it's gone past you let the slow one follow the fast one again.
You can use one controller to power two loops. You could either have two totally independent loops or have them connected by points in some way. If you want independent loops you will need an extra #5305 9V Train Connecting Lead to get the power to it. Since most people don't have extra leads, however, you probably want to go with the points method. Points can route power to another loop as well as allowing the trains to swap over for a bit of variety. This is explained in more detail in the section on How To Wire Up Layouts With Points. But if you just want to see an example, look at Euro Two Track.
For really long trains you might consider putting two locomotives at the front for extra pulling power. You might think that this wouldn't be advantageous if you imagine that the power is getting shared between the train motors, but actually the motors are effectively connected in parallel. They demand more current from the controller and it is perfectly happy to provide it.
Some of the train sets lend themselves nicely to upgrading to twin motors. This is an alternative to a double headed train. #4558 Euro Express can easily be modified by adding a motor unit to the unpowered end and #4564 Freight Express can take two motor units on the locomotive. This is a good way to get more traction if your layout has steep inclines in it.