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. 

New York City Subway - Wikipedia

11 - Rail Crossings Ohio Department of Transportation

Why is the board on the side of railway tracks written W/L and how

What is the meaning of the W/L {सी/फा} sign board written near

Railway Signals and Sign Boards Meaning

100+ Train Images Download Free Pictures On Unsplash

What is W/L and W/B Sign Boards On The Side of Railway Tracks

How to use SEPTA to get around Philadelphia

Did You Know the MTA Uses Pantone Colors to Distinguish Train

NS Norfolk Southern

Indian Railways Sign Boards and Their Meanings

Railroad Signals (USA): Explained, Meaning, Examples

Railway Signals and Sign Boards Meaning