'The Eighth Doctor isn't satisfied with his model train set any more, as it was built by his seventh incarnation as a model of clockwork timing, and that just isn't him any more. Instead he re-engineers the entire set to work by itself without needing his constant attention and is delighted by the complex patterns that result. Unfortunately, after leaving it alone for a time he returns to find that a group of impatient clockwork passengers have started a chain reaction which has resulted in the total destruction of the train set. But a few of the clockwork engineers have survived and are beginning the process of reconstruction, and the Doctor helps to get them started, still hoping against all the evidence that this time they'll get it right.'
Source: drwhoguide.com (http://www.drwhoguide.com/whotrip1.htm). Sighted 20/5/11