Knapford Harbour began as a wharf around 1885, connected to another wharf at Elsbridge by the lightly built, horse worked Knapford and Elsbridge Railway. It initially served as a base for A. W. Dry's drainage operations in the area, but eventually proved a disappointment. It needed constant dredging and by 1905 the cost had become prohibitive. They abandoned it and extended the line along A. W. Dry’s road to Tidmouth to use the port there. From then until 1956 the harbour remained little used.
By the mid 1950's, there was severe congestion at Tidmouth, so Sir Charles Topham Hatt launched his Knapford Harbour Improvement Scheme, using plans created by his father, to redevelop the harbour and relieve the congestion at Tidmouth. Percy was transferred from Tidmouth to assist, while Trevor was loaned from the Vicar of Wellsworth. The work caused considerable realignment of the entire branch line and was finished in 1957.
Knapford has been useful as a supplementary harbour, but its exposed position did not enable it to provide the best answer to Sir Topham Hatt’s problem. That was why in 1966 he went north to see what could be done at Arlesburgh.
The southern quay is accessible from the goods branch stemming off from Thomas' Branch Line in Dryaw. According to the Thomas' Branch Line Surprise Packet Map, the northern quay has a spur from The Main Line, going over a bridge in front of the main line one and into the northern quay. However the two quays are connected by a small bridge crossing the river which is not present on the book illustrations, but it is present on Sodor maps. There is a small railway station located by the docks, separate from the main line station.
- Big Mickey and S.S. Vienna's models from TUGS were seen at this harbour during the third season, but since the fifth season they have been seen at Brendam.
- In the second season, the new harbour that appears in Thomas and Trevor and Percy and Harold is different from the one that appears throughout it. It is possible that both are different areas from the harbour, or the harbour shown in those two episodes is actually Brendam.