13 carriages is the absolute limit for any passenger train on the FR, in normal circumstances, due to the length of passing loops. We don't usually go beyond 11 during the main season, when several trains are running, as some loops are shorter than others.
The spelling is Merddin Emrys - IIRC it's the Welsh form of Merlin, the legendary Mediaeval wizard associated with King Arthur and the Knights of the Round Table.
