UP6936-great question and there are a number of answers to it that some of the guys have already mentioned above.At the start of the Hunslet operation in 1970,NIR used the Hunslets top and tail with all 8 coaches which comprised 2x std driving trailers,1x first open,4x second opens and the buffet car.This soon reduced to 7 coaches with 1 coach held in reserve,normally a second open.A reduced winter formation was also used as mentioned by David,which was 1 Hunslet with up to 5 or 6 coaches in push/pull formation,again this would have had the std driving trailer,first open,buffet and 2 to 3 open seconds.6 coaches seemed to become the norm,although this was a tad heavy for 1 Hunslet.In 1972,more coaches were purchased,1 of which was a 1st driving trailer,which thus meant the winter formation started to comprise of it,the buffet and 3 std opens,which was a bit more manageable for the 1 Hunslet.As with all things on NIR,this was never set in stone...With the extra coaches,a 10 coach set and a 12 coach set were tried,but this was not a success and was soon dropped.As Colin pointed out,the driving trailer was normally at the Belfast end as this suited facilities at Great Victoria Street,but again there were exceptions to this also...Hope this helps.