I tend to see confusion here about the harbours in the TV series when I ramble on about the Sodor map, so I decided to make a thread explaining each. Here's a TED Talk on Sodor's harbours.

A thread:

First, Railway Series.

Tidmouth Harbour is the biggest port on Sodor in the books. Every time they go to "the harbour at the big station", it's this one. Henry and James collected the Kipper here.

Knapford Harbour is the little harbour Thomas' branch services. It was closed when Tidmouth Harbour became the island's main port, but Topham reopened it in Percy the Small Engine, hence why Percy helps rebuild it. All harbour stories from Thomas' branch line occur here.

Brendam is a tiny harbour in the books that Bill and Ben bring clay to. It only appears in the 2 Bill and Ben books.

Arlesburgh Harbour is on Awdry's map, but it never appears in illustrations. Mid Sodor used to service it, it closed, and then reopened for service when Topham opened Duck's branch.

All pretty easy to understand, ye?

Now for the TV series. This is where things get confusing.

Knapford replaced Tidmouth in the TV series, so naturally Tidmouth Harbour became Knapford Harbour. As far as we've seen, Tidmouth Harbour never appears in the show and it may not even exist.

The big harbours in Seasons 1 and 4 are Knapford. In Fish, it's directly called "the harbour at the Big Station by the sea" aka Knapford. Main line and Thomas' branch both service this. Henry collects the Kipper here, Stepney collects stone here, Percy loads Bulstrode here, etc.

Technically Knapford appears in Something in the Air too, even though it is just the Brendam set redressed. I only say this because it looks like they tried to recreate the Fish setup with the warehouse next to the tracks to be consistent - it's not there in other episodes

The old harbour Thomas and Trevor rebuild is also Knapford, but it's an older sect of it that was previously closed south of the River Els. This is only serviced by Thomas' branch. Clearly modelled after the Knapford in the books.

There is no Elsbridge Harbour, that's dumb.

The big harbours in Seasons 2 and 3 are all Brendam. Brendam is big in the TV series. Bill and Ben come here on multiple occasions, Percy and Duck worked here for a while, and the sunken track Percy plunges on gets rebuilt in S3. The concept art calls it "Brendam".

I admit there are some weird continuity errors with this. Gordon collects the express at Brendam in Heroes which makes no sense (special night charter train maybe?), and Percy and Thomas incorrectly come here in Plunge.

Season 5+ is also Brendam, but it looks totally different to S2-3. Since they completely threw the continuity of which engines work here out the window, we have to just be willing to shrug off the major layout differences. (Frustrating, I know)

For some dumb reason, Brendam becomes the Island's main port in Season 5 onwards. I have no idea why (is it because the name is different than Knapford so they can market it?)

Whatever, Brendam's the main harbour as of current day and it makes no sense

Arlesburgh Harbour never appears in the show proper until Lost Treasure. Since it never appears in the books either, this is its first appearance ever.

So, in conclusion, in the TV series:

-Knapford (S1, 4, and Something in the Air)
-Knapford old south harbour (S2 - Thomas and Trevor, Percy and Harold)
-Brendam (S2 and 3) (Redesigned S5+)
-Arlesburgh (CGI only)
-Tidmouth (never appears)
-Elsbridge (doesn't exist)

I hope this was helpful for some, cus this shit's confusing. I know for some this is obvious stuff, but I wanted to make this thread regardless. Please discuss!