Currently it appears to be at a stand in the Old Oak Common area. From what I can gather from Twitter accounts of people I follow who are stuck on it, it's failed with loss of air and a fitter is awaited. (
RTT» suggests that the 18:10 HEx is stuck behind it.) Understandably there are complaints about the temperature inside the train and @_OzBurns has posted this picture:
UPDATE: Looks like the train is now on the move again, some 65 minutes late. (and HEx tweeted that their 18:10 returned to
PAD» .)