Permanent track speeds are designated in special instructions that specify the authorized speed and what other element?

Answer: A — Milepost limits of the speed.

Permanent track speeds are tied to a specific segment of track defined by mileposts. The Special Instructions list the allowed speed and the milepost limits that mark where that speed applies. So the speed is valid only between those two mileposts; outside that stretch, a different speed applies. Weather, train type, or brake-test intervals don’t define the boundary of the speed, they may influence how you operate, but the milepost limits are what locate the authorized speed. For example, you might have 60 mph between mileposts 200.0 and 205.2—within that range you must not exceed 60 mph, and outside it you follow the applicable other speed.