Temporary speed restriction
Also: tsr, speed restriction, slow order
A lower speed limit put on a stretch of line for a while, often for track work.
When track needs attention, or has just been renewed and is settling in, trains may be told to slow down over that stretch until it’s fixed.
Network Rail publishes the list of them. A train slowed by one can lose a minute or two, which adds up if there are several on a route.