This is about the first time that I see "UK" and "great train service" together in one sentence ;)
I define great as it being generally possible to get where I want to go on the train (this is the most important factor) and then as a bonus it's usually competitive with driving, busing, flying, cycling, or walking in terms of comfort, cost, and convenience.
When people praise train travel it is largely long distance.
When people complain it is likely to be commuter lines in and around cities.
BTW, TGV is French...