Hi unittrain, I am no expert but I will try to respond to your questions. My layout is PRR, 4 track main line, double level, 250 feet of main line and I run a Digitrax Chief with a Booster or Slave unit. Typically there will be up to 20 locomotives in motion at any one time. That gives you a little context of size of layout vs Command Station. With a 60 foot main line you will only need a single Command Station. If it is Digitrax it will probably be 5 amps. This should be ample for the number of locos you want to run. Your double track mainline has no bearing on the number of Command Stations. The same would apply if you used NEC, MRC, or any other manufacturers Command Stations. Best advice I got on which one to select was "determine what the other guys in your area are using, and what your hobby shop supports, and go with that". They theory being if you need help there are others around.
In any event, your questions suggest a beginners level understanding of DCC. (if I'm wrong, my apologies). If so you might want to check out this website.
http://www.wiringfordcc.com/wirefordcc_toc.htm Great stuff for everyone from the beginner to the expert.
Hope I helped and pointed you in the right direction. Once you have looked at Allan Gartners info, ask any specific questions you still have and I'm sure I or someone can pitch in and try to help.
