Notarzt, in answering your question the TriStars are now only used to LHR. With the 737-8Q8s having mostly taken over the North American routes on which TriStars were flown (JFK, YYZ) BWIA decided to keep only 2 TriStars in service, with one plane at a time active and the other as a backup. At peak periods, however, the TriStars would be used into JFK to accommodate the great passenger and baggage loads typical on the route (which, especially the latter, have already shown the 737-8Q8 to be inadequate).
9Y-TGJ was retired because it needed a C-check and corrosion repairs, unlike the other TriStars.
Hop to it, fly for life!