From various IRI Blog Posts:
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The fouling point or fouling mark is a point to the rear of a converging junction, such that a train must be to the rear of that point in order to ensure that any train moving on the other converging line can proceed without being obstructed.
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It...
more... is the point beyond the converging point of two or more tracks before which train on one of those tracks have to be stopped so that train movement is not obstructed on other tracks.
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FM stands for a fouling mark, no train can cross this point without being permitted in some form.When a long goods rake is parked in a loop, the guards exercises caution to make sure the total rake has crossed the FM point.
This sign is fixed at a distance required at the minimum to ensure sufficient distance of rake from the next track even though the train has vacated the earlier track.
Railfanning Note: Observe the FM mark next time, you can usually find the track(loop) curves to join the mainline after this point.