Until the full withdraw of loco hauled services some platforms were too short to accommodate a full length main line train, as a result some carriages used to be off the rear of the platform. This short platform, long train scenario could be used to make the train seem longer.
Also not all mainline trains were very long, long gone are the days of 13 coach Friday only specials, I have traveled from Limerick to Dublin on various setups
4 Push pull and loco (As far as I remember they were limited to a set of 6 anyway)
3 Mk 2s and a van
Short 6 piece Mk 3 sets
2 piece 2700 relief service
5 Piece Mk2 sets