Using Bachmann EZ track, what's the rule, or need for feeder wires?
Ideally, every rail on the layout should have its own feeder. Even E-Z Track suffers from voltage drop and the joiners can loosen.
But for those of us who can't afford to make an E-Z Track layout from only terminal rerailers, let's just say 'The more the better'.
I placed them every 3-5 feet on my layout. Works much more reliably than when my controller was powering 70 feet of track from a single feeder.