Arnold,
You'll get plenty of different opinions on this but I keep my ground (or common) lines separate for each of my main lines. Thus both the hot and common wires, sometimes twisted together, go from the separate TIU output terminals (Fixed 1 and Fixed 2, for two main lines), via breakers, out to the track feed points. There are some technical reasons to do this, some related to troubleshooting and also for common (mode) noise rejection. Arguably it offers some advantages for the MTH DCS control system.