I think the tracks for slideways and mobile cranes can hardly be called "railway", a harbour railway must at least had freight (at most) traffic from other places.
Being connected to a wider rail network probably isn't necessary, since one could imagine a large port complex with significant internal traffic being otherwise disconnected from the main line railway. I don't know if Gibraltar dockyard had an internal rail network, but if so it would qualify here. Of course, such a harbour is also unlikely to be small.
Good shout that, but didn't the OP specify standard gauge?
Interesting... both https://maps.nls.uk/view/75679737 (1907) and the 1897 one show a cutting from Portpatrick station towards the harbour but no track. I thought it might have been under construction in the first one, but it seems track was not laid.
Or maybe by the mid 1890s, the harbour branch line had been abandoned already? Possibly closed for several decades by then.
So, possibly so short-lived that the trackwork for the Portpatrick Harbour branch never actually appeared on an OS map revision?
True....but that doesn't usually stop people on here from going off at a tangent, despite the forum rules!
That's what I thought about when I suggested Wells-Next-the Sea. Not a pretty pair of walls enclosing a sheltered bit of water, but still the way that most longer-distance freight moved to anywhere vaguely near the coast right up to the time the railways came - unless there was a canal within reach.
This line was spectacularly awkward, accessed by a training junction beyond Portpatrick station from a very short shunting neck, with a capacity (according to D L Smith) of no more than a loco and two four wheeled carriages. The ruling grade was 1 in 35 just to add to the fun! The line was lifted in 1875 by the CR for the princely sum of £14 10s 6d.
but surely Sharpness, Ayr, Troon and Irvine can't possibly be anywhere near "the smallest harbour?"
I would say that there was definitely once a rail connection down to Tweed Dock (Tweedmouth) from the ECML mainline, likely needing a double reversal to zigzag its way steeply down the hillside from Tweedmouth station. Believe the dock railway line was operational from c. October 1878 until the mid 20th century.
This is a distinct possibility - I was in a pub at the harbour which had the usual ‘olde worlds’ pictures of the area, and one of them showed a train in steam reversing (I believe) down to the Harbour and the waiting ships to NI (which included mails) until the unsuitability of the harbour in inclement weather switched them to Stranraer.
The standard gauge Cainryan Military Railway branched off the main line between Dunragit and Stranraer. Military Port No 2 at Cairnryan was constructed in some secrecy in 1940 to provide an alternative deep water dock should Liverpool or Glasgow be rendered unusable by enemy bombing. The installation ended up with 30 miles of track, sidings for 2,900 wagons, 4 harbours, 31 wharf gantry cranes, 4 stations and accommodation for 4,000 men. So not really a contender as the smallest harbour, I suggest.
