Large Scale Central

Power calculation

My name is Rick Lucas, I am interested in starting a large scale railroad. I have a question that I have taken to my local hobby shops with no answer. How can I figure out how large of a power supply is needed for what amount of track. For example if I have 500 feet of track how large of a power supply will I need? I would be looking at a minimum of 2 locomotives with cars as well as lighting for signals and other accessories. Any assistance would be greatly appreciated.

Rick