A turnout defn, in a .xtp file, contains segments which are simple pieces of straight and cureved tracks.
There can also be non-track segments like lines, rectangles, polygons etc.
There are also Paths in the defn (P lines) which define the seguence of segments to get from one endPoint to another.
There error is saying the segment 3 (which is a track) doesn't appears in any P line.
This inidicates that something is wrong with the defn
The line number (374) is actually at the end of the turnout defn.
Please attach the .xtp file in question and let us know where you got it.