Which signs tells you to watch out for a train?

A black and yellow circle that has a R on the right, and a R on the left with an X in the center of the circle. Page 5-18

There normally arn't if you're driving, its a hazard sign which is a triangle with a red boarder marking a level crossing that tells you you're near train tracks (UK)

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In the US, it is elevated cross arms in the shape of an X, white, with STOP, LOOK, LISTEN painted on them. The crossing MAY have flashing red signal light, and MAY have an automatic arm that blocks the roadway.