Block Limits
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For stations (usually class 'B') on double line sections in absolute block or automatic block territory (and usually with modified lower-quadrant, upper-quadrant, or MACL signalling) a square or rectangular yellow sign with two intersecting black diagonal stripes, and with the words 'Block Section Limit' is provided 180m in advance of the Home signal; it marks the fouling point of the rearmost trailing points connecting to the station.
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more... The acronym 'BSL' may appear instead. The legend faces the station.
A single white lamp may appear within the sign through a hole in the sign above the center.
source:IRFCA