[11][12][13] Passenger carrying did not begin until the railway (sometimes referred to as a tramway) had been running for 28 years,[14] but it nevertheless made a significant contribution to income, e.g. over a quarter in 1862,[15] but its profitability was another matter, particularly as it risked interfering with slate traffic.

[18] That of August and September 1856 shows three passenger trains taking 1hr 30mins southbound from Carnarvon Castle to Nantlle and 1hr 21mins northbound, the difference being caused by the prevailing gradients.

An additional stop, five minutes before arriving at the Castle terminus, appeared in October 1857, northbound only, advertised as "Carnarvon"; this disappeared soon afterwards never to reappear.

Different class travel was provided, but the only shred of surviving evidence of what that meant can be gleaned from a press report concerning the line's sole passenger accident, which occurred near Bontnewydd in June 1861.

In a forerunner of modern times the track was owned by and the responsibility of the Nantlle Railway Company, but the services were run by a private supplier - Edward Preston - acting under lease.

[29] Eventually a branch was built at the Caernarfon end from the standard gauge line to the quayside, rendering the 22 yards (20 m) tunnel at Coed Helen[30][31] and the Nantlle's bridge across the Seiont redundant.

The first action by the LNWR (who had taken over the Carnarvonshire railway[32]) was to build a trailing junction a short distance from the southern mouth of Caernarvon Tunnel, not far from the future site of the modern Caernarfon station,[33] leading into St Helens Road, where goods were transshipped to former Nantlle wagons to be handled on the quay.

The PBSSR's life and history is complex, but two of its enabling Acts made provision for extensions from the northern end of the NWNGR from Dinas to the quayside at Caernarfon.

The Act of 15 August 1904 authorised a line northwards from Dinas running near the LNWR's Caernarfon to Afon Wen line then veering westwards through the erstwhile Nantlle Railway's Coed Helen tunnel then crossing a new bridge approximately on the site of the original Nantlle Railway bridge over the Afon Seiont terminating on the quayside near the Harbour Offices.

In the application to Parliament (which had been made as long ago as 1905) the estimated cost was "£3241...with £500 allowed for a station located beneath the castle walls.

The LNWR supported all moves to build a unified line northwards from Porthmadog, but opposed a northern narrow gauge extension from Dinas.

The Commission's finding was to support the formation of the Welsh Highland Railway which would join the PDSSR and NWNGR to give a through route from Porthmadog to Dinas, but the northern extension to Caernarfon was not backed and thereby died.