What is "tracker action"

Tracker action is a term used in reference to pipe organs and steam calliopes to indicate a mechanical linkage between keys or pedals pressed by the organist and the valve that allows air to flow into pipe(s) of the corresponding note. This is in contrast to " direct electric action" and " electro-pneumatic action", which connect the key to the valve through an electrical link or an electrically assisted pneumatic system respectively, or " tubular-pneumatic action" which utilizes a change of pressure within lead tubing which connects the key to the valve pneumatic.