The line was built about 1800 to reclaim marsh around Tremadoc by William Alexander Madocks. He founded the town at Tremadoc and extended his tramway on to Portmadoc (named after him) and then further across the cob to Boston Lodge.
The Glaslyn was scoured and Portmadoc built there in 1821. This became the port for export of local slate.
These locations are along the line.
A Regional History of the Railways of Great Britain: North and Mid Wales v. 11 (Regional railway history series)
Porthmadog Ships (Morol)
Portmadoc and Its Resources 1856
Taste of Madocks: And the Wonder of Wales
Tremadog 1913: Caernarvonshire Sheet 34.12 (Old O.S. Maps of Caernarvonshire)