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. 

100+ Train Images Download Free Pictures On Unsplash

The Making of the Perelman Performing Arts Center

System Facts Bay Area Rapid Transit

What does W/L represents on side of the rails? - Quora

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

Indian Railways Sign Boards and Their Meanings

What's the W sign I see train workers hold up sometimes when a

Figure 8B-6. Example of Placement of Warning Signs and Pavement

What Is a Bill of Lading? Definition and Guide (2023)

Bill of lading definition: What is a BOL form?

Railroad signals

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

Hazmat Labels, Hazmat Placards, and Hazmat Markings - A Guide

High Visibility Railway Sign - China Railway Sign, Railway Signage

Chapter 8B - MUTCD 2009 Edition - FHWA