My thoughts.
There are two level of electrical performance when dealing with unsoldered rail joints.
Level 1: The best performance can be achieve if you do not rely on ANY metal rail joiner for electrical conductivity. You will not lose power regardless of the electrical condition of the metal rail joiner. Stated another way, the rail joiner is only needed for mechanical alignment of the rail. This is called a "Double Fault" tolerant system because both rail joiners can electrically fail and you will NOT lose power on any track. If you lose a track feeder, then the section of track it feed will by default fall back to a level 2 power setup and will still get power unless both rail joiners are each end of it are bad.
Level 2: An OK service can be achieve if you have a feeder installed with no more than two metal rail joiners between them. Another way to say this is every other section of rail gets a feeder. The section of rail between feeders will depend on the rail joiners on each end for electrical power. This is called a "single fault" tolerant system because one rail joiner can electrically fail but the track in the middle will not lose power. The rail will only loose power if BOTH rail joiners electrically fail (AKA lose and/or oxidized). However if you lose a track feeder, the middle section of rail will now depend on multiple rail joiners for track power on the failed feeder end. The odd of electrical failure go way up as you depend on more and more rail joiners for track power.
A insulated rail joiner only electrically separates the rail to support 2 different track power circuits (Blocks). An insulated rail joiner from a electrically reliability point of view can be viewed as a intentional pre electrically broken rail joint removing potential path for track power to reach the dead section of rail in the middle. Therefore the rail on each side of the insulated rail joiner should get a track feeder by default.
The 6 foot feeder rule is a arbitrary one in the sense it is not taking into account the potential electrical failures nor the type of track being used.
The 6 foot rule:
1) Offers Level 2 performance if and only if your using 3 foot section of Flex track.
2) Sub par performance if you have MORE than two rail joiners between feeders.
A 3 foot rule.
1) Offers Level 1 performance if and only if your using 3 foot section of Flex track.
2) Offers Level 2 performance if there are only two rail joiners between feeders.
3) Sub par performance if you have MORE than two rail joiners between feeders.
Unsoldered Rail joiner electrical fail due to oxidation. When the rail/track moves due to roadbed expansion and contraction (humidity and temperature changes) , the rail joint is the place where the movement is seen. This allows the rail joiner to slide and expose the rail and the rail joiner to increased oxidation. At worse the rail joiner is partially deformed under the movement and wiggles lose such that it is no longer able to maintain metal to metal contact under pressure. AKA lose contact = poor contact. At a minimum a lot of resistance is created but some current flows to complete failure were no electrical current can flow.
12/19/2020