Do any Class 166s operate on this route?
The standard train is supposed to be a 165 + 166 I believe - total 5 cars - though at present many are running 3 car and that has been plenty in very recent times.
Oh - sorry - took your "this" to be a follow up to you Portsmouth - Cardiff question.
RIGHT answer - "Yes" - tends to be mostly 165 with the occasional 166. The 07:03 Westbury from Westbury is scheduled to be a 3 car train (that makes it a 166, right?) because of passenger numbers later on its route. Last autumn when schools were in, it got cut to 2 cars one day and became the most overcrowded train on GWR with a frightening overload number!