I use 4-track for 2 lines:
1 line going north to south, the other going south to north.
Line 1 stops at every stop, till it reaches the south station, then it goes on the middle track, immediately back to the north station.
Line 2 does the same, but then from south to north.
This system allows for immediate peak-coverage, and the return trip is very valued by the CIMS(because you don't need to stop EVERYWHERE).
What a great idea. I'll have to try that.