* Axle Counter – track-mounted equipment which counts the number of axles entering and leaving a section of railway (US: railroad) track, to determine whether it is occupied or free.

* (Axle) Counter Head – a device underneath a railway track, and linked to the Axle Counter, that detects axles as they pass over it.

* Axle-Counting System (ACS).

More detail if required:

* An axle counter is a device on a railway that counts the number of train axles passing over a section of track.

* A counting head (also called a 'detection point') is installed at each end of the section, and as each train axle passes the counting head at the start of the section, the (separate) axle counter increases by one. As the train passes a similar counting head at the other end of the section, the counter decreases by one. If the axle counter calculates that the net count is zero, the section is presumed to be free (unoccupied) for the following train to enter it.

* An axle counter can detect not only the number of axles passing over the counting heads, but also the direction and speed of the train by the order and time in which the heads are passed.