Proteus is a yellow ghost engine who haunts Sodor.
Not much is known about Proteus before coming to Sodor. But when he arrived he worked in the mines with Duke and Smudger, before Smudger had been turned into a generator and put at the back of the MSR Sheds.
Before Smudger had been moved to the mines as a pumping engine, before breaking down and mines flooded, late one night Proteus had been working in the mines, when a terrible rockslide occurred completely burrying Proteus, when he was found the next morning William Meisner and a young Bertram Hatt decided not to recover him, and as a result the Mid Sodor Railway was closed down, and both Duke and Smudger were abandoned. And when the tunnel connecting the Skarloey Railway to the North Western Railway was made his remains were disturbed and taken to the scrapyards.
However it seemed that Proteus had unfinished business as his ghost seems to return and controlling Willam's mind having him write the same scroll over and over, until it drove the controller insane and he committed suicide after he left work, leaving Bertram to inheriting the railway.
It seemed Proteus still wasn't done as one day in 1945 as Skarloey was making his way back to the sheds, his ghost appeared frightening the old engine to no end. And when the engines were reminiscing about Lady and how time had gone, the engines heard Proteus' whistle in the distance, this caused Skarloey to tell the others what had happened, this prompted Duke to tell the others about his old friend, right before he got captured by Andy and the other diesels in the night.
A few nights later the engines hear Proteus again, Sir Handel having enough goes out to confront Proteus, when Proteus appears he tells Sir Handel that he wants his remains returned to the mines so he could slumber once and for all. After that Proteus continued haunting Sir Handel for years after the diesels were sent back to the mainland.
During Stephen Hatt's control over Sodor, Proteus appeared again in the Old Mine after Angela had read one of Proteus' scrolls. After Proteus finished his poem, he had his lamp light up the whole shaft, causing the mines to collapse again, killing Angela and Don, and trapping Bertram who had witnessed the whole thing, after the brown tank engine was taken to the Steamworks to be repaired, he told the others what had happened. Sir Handel finally gets the courage to tell the others about Proteus filling him around, wanting him to do something. Eventually Sir Handel tells Phil Aardman about Proteus and requested to bury his remains, which was done and the yellow engine could rest in peace.
Proteus may still be among the engines as during the events of "Mid Sodor", Robert Stewart found an old headlamp, but dropped it and broke it when he saw something else, and Proteus' whistle was heard again. It is unknown if Proteus is really at peace or not.