Caution Indicator
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Arrow-shaped boards pointing to the left or right. These indicate special restrictions on the track (temporary or permanent engineering restrictions) and caution orders in effect; the direction of the arrow indicates which track the restriction applies to. These boards are usually reflective yellow with black markings. The post on which it is mounted has alternating black and white bands. At night, sometimes two yellow lamps illuminate this indicator, although such lighting is not common and perhaps used more for caution indicators that are installed permanently. More often, the reflective...
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paint on the indicator suffices for visibility at night. The caution indicator is usually placed 700m before a Speed Indicator board (see below) and 800m before the actual point of permanent way work or other cause of restriction. Drivers have to slow down to the speed indicated on the speed restriction board by the time they reach it.
Other indicators include 'CP', or 'CG' on small white circular boards. These are caution indicators for passenger and goods trains, respectively. A 'C/T' indicator has been spotted in a few cases just before the entrance to a tunnel.
source: IRFCA