The idea of using the crossovers came to me when I decided I didn't want to have a regular folded type loop mainline. I've always loved the look of a double track main so my layout is a reflection of that love. Using the four 60 degree crossovers allowed me to literally double the length of the of the double track mainline.
Here is how it works. Referring to the picture of the crossovers (I've attached it again here so you don't have to page back to the other photo) you'll note that two are let's say north and south and two are east and west!
Your train is on the left northbound track and stays on this track until it circles the room on the entire upper level. When it enters the diamond for the second time it will enter on the upper west bound track. This track starts to go down to the lower level to the right. When the buildings are complete the train will slip behind the buildings. Additionally, if your train is on the right northbound track it will enter the diamond for the second time on the lower westbound track. Are we confused yet?
The two tracks going west and then north down to the lower level will circle the room on the lower level and come up on the opposite side of the room and back to the starting point on the diamonds.
With two trains running, this is no problem but you still need to pay attention to the crossovers as your trains will be twice as likely to meet. When we run four trains you have two on the upper level and two on the lower level. The diamond crossover is now four times as dangerous.
Now, let me throw a little spike into the mix. If any one of the trains crosses over to one of the others tracks, you will have three trains on one main line and now you can see the problem. This is why JMancuso mentioned the crossovers as a point of some self induced stress while running trains. However, we all have been caught shooting the bull and BSing and had cornfield meets at the diamond!
Now, the above scenarios are only if the trains are running on one direction. We will run them in opposite directions as well for more fun at the "Four Diamonds".