‘W’, or ‘W/L’ on a square yellow board. The ‘W’ is a general whistle indicator while the ‘W/L’ stands for Whistle for Level Crossing. The latter is also seen in Hindi with the characters ‘see/pha’ == ‘seetee bajao – phatak’). Usually provided on approach to unmanned level crossings (and for manned level crossings without a clear view) about 250m away from the level crossing. Similarly, a ‘W/B’ sign is seen (less often) on approach to a bridge.