What do 'W/L' and 'W/B' signboards on railway tracks mean?

All of us must have enjoyed the train journey. Many of us are regular train commuters. While travelling, we must have noticed several letters and signs printed on coaches or put up by the side of the tracks. Most of us are not aware of their meanings. 

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W/L' and 'W/B' Signboards on railway tracks means

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