The S and S2 Classes that had previously served the route were giving trouble as boiler pressure had been raised to increase power and performance.

This increased maintenance (particularly with broken crank axles) and as a result the boiler pressure was brought back down.

The GNR named the locomotives after birds of prey: 83 Eagle, 84 Falcon, 85 Merlin, 86 Peregrine, and 87 Kestrel.

They had a round-topped firebox and Stephenson valve gear and weighed 103 tons 11 cwt including tender.

87 Kestrel was the first to be rebuilt in 1946 and was the first GNR loco to receive a Harland and Wolff-built square topped Belpaire firebox.

Merlin spent its final years as a spare in Dundalk works, occasionally hauling a Dublin train.

Displayed here until 1976, it was moved to Harland and Wolff for a comprehensive overhaul organised by the Museum, the Railway Preservation Society of Ireland (RPSI) and Lord Dunleath.

Itrepeated the trip on the 40th anniversary of the Enterprise the following August, hauling the same length train as in 1947 (seven bogie coaches) and matching the timings of the original run.

Having received a further overhaul in the 1990s, Merlin continued to be leased by the RPSI and used on the main line until its boiler certificate expired in 2004.