The issue you face is less about the TIU and more about how much power you need on each of the 5 tracks. Do you have a sense for how many engines you will be running simultaneously and how long of a train and whether the cars are lighted? Each of the two main Z4000 handles give you 180 watts of power. That is 10 amps each. You can't max these out without throwing the breaker on the Z4000. The most I have seen a train pull in command mode is about 1.8 amps. You could certainly pull more with multiple engines in a lash-up and lighted cars. However, you could probably run 4 engines per handle safely. It doesn't really matter too much how many tracks you have as long as they are blocked and the power to the trains is distributed evenly.
George
