Most standard track cogs have a right handed thread (1.37"x24tpi). They are rotated clockwise to tighten onto a fixed gear hub, and rotated counter-clockwise to loosen it from the hub. On top is a reverese threaded lockring to secure the track cog in place.
A new track cog arrive yesterday and was installed onto a Surly 135mm (MTB Spacing) Rear Disc Fixed Hub. Posted is an article with instruction on removing and installing a track cog, which turned out to be bit more difficult than anticipated. The old cog would not rotate off the hub! After a few attempts a simple solution was finally found.