If you read to the end of the article you see: "Finding replacement parts won’t be a problem and DOS is no longer necessary for Allen and his crew, but it will

If you read to the end of the article you see: "Finding replacement parts won’t be a problem and DOS is no longer necessary for Allen and his crew, but it will still be bittersweet to say goodbye to trains that they have lovingly kept alive." So they don't actually use DOS anymore, maybe, or possibly this just means they don't care because the new trains will soon replace the old ones and then DOS will be gone too. It's a poorly written article even allowing for the usual tech mistakes but either interpretation would be reasonable.