31 Collectable Model Trains: Tracks: Why are three rails often used?
Description
This article is from the Model Trains FAQ, by Christopher D Coleman with numerous contributions by others.
31 Collectable Model Trains: Tracks: Why are three rails often used?
The principal problem with two rail track is that the two rails
contain opposite polarity voltage. When the tracksloops back on
itself the opposite rails will meet and a short will occur:
___B_______________________________B_______________
\ \
_________________\______________________________ \
A \ \ A \ \
\ \ \ \
B\ \A | |
\ \ | |
\ \ A/ /B
\ \___________A____________/ /
\ /
\______________________________/
B
In three rail the outer two rails carry the same polarity with
the inner rail opposite. Shorting is not a problem:
___A_______________________________A_______________
___B_________\_____________________B_____________ \
___________\__\________________________________ \ \
A \ \ \ A \ \ \
\ \ \ \ \ \
A\ B\ \A | | |
\ \ \ | | |
\ \ \ A/ /B /A
\ \ \_______A__________________/ / /
\ \_________B____________________/ /
\__________________________________/
A
This allows the construction of much more complicated layouts
without electrical shorts. It also allows the insulation of one
outer rail for the purposes of powering signaling accessories
without disrupting current flow to the train and without the use
of clumsy pressure plates.
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