fouling markn.South AsianRailways a sign or notice indicating the point at which the distance between two converging railway lines falls below the minimum required to avoid collision or obstruction (cf. sense 2).
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1900 Indian Engin. 30 June 404/2 The starter should be at the fouling mark of the main points.
2016 @vadakkus 5 Nov. in twitter.com (accessed 3 Mar. 2021) The facing train on the joining track seems to have gone beyond the fouling mark. Driver lapse.