What is the most economical way to run multiple trains on n scale layout?
The most economical way may not be very interesting.
But if that is all you are concerned with, I would build two loops of track that are not even connected with each other. Perhaps one closed loop inside the other.
Put a train on each loop. If you use a separate power pack for each loop, you can run trains at different speeds and directions. If that costs too much, then wire them both to the same power pack, but they will run in the same direction unless you reverse the power connection to one track loop. If you do this, make sure your power supply can handle two trains.
There you have it. This would work well for something like a store window display.
Les
Probably a basic block system is the least expensive control system for model railroads. Here is a link that should help get you started.......Mike
http://en.allexperts.com/q/Model-Railroads-2372/2009/3/Running-2-trains-set.htm
You basically need a digital command system (EZ Command item no. 44902) and decoder equipped locomotives.
Well, there you go.
Three good answers and the three ways to do what you want.