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Along with her sister ships the TSS Duke of Rothesay and the TSS Duke of Argyll she was amongst the last passenger-only steamers built for British Railways (at that time, also a ferry operator). She was a replacement for the 1928 steamer built by the London Midland and Scottish Railway, RMS Duke of Lancaster.
Built at Harland & Wolff, Belfast and completed in 1956, she was designed to operate as both a passenger ferry (primarily on the Heysham-Belfast route) and as a cruise ship. In this capacity, the Duke of Lancaster travelled to the Scottish islands and further afield to Belgium, Denmark, Netherlands, Norway and Spain.
From the mid-1960s, passenger ships such as the Duke of Lancaster were gradually being superseded by car ferries. Rather than undertake the expensive option of renewing their entire fleet, British Railways instead began a part-programme of conversion. In order to maintain ferry services whilst these modifications took place, the Duke of Lancaster's duties as a cruise ship ceased. On 25 April 1970 the ship returned to service, having had her main deck rebuilt to accommodate vehicles via a door at her stern. The ship now provided space for 1,200 single-class passengers and 105 cars, with a total cabin accommodation for 400 passengers.
The three ships continued on the Heysham-Belfast route until the service was withdrawn on 5 April 1975. The Duke of Lancaster was then briefly employed on the Fishguard-Rosslare crossing, before becoming the regular relief vessel on the Holyhead–Dún Laoghaire service until November 1978. The ship was then laid up at Barrow in Furness, Cumbria.