I'm still searching for that Ballarat fly.  Miriam Jones Shillingsburg
speculates that he found the name of the train while reading train
schedules in Napier, New Zealand but the only name for that train I've
found is the "Napier Express".  I've also searched for that energetic
green fly he saw in this hotel room, to no avail.  I've tried inquiring
both train specialists and entomologists to no avail.  The only
Ballarat fly of record is a fishing lure created in Ballarat,
Australia.  As far as I've been able to tell, Twain never showed any
interest in fly fishing.
-- 
 There are more things in heaven and earth, Horatio, than are dreamt of
                          in your philosophy.
                        http://bscottholmes.com