Complex sentence: They were the first US-built (and the privately owned) diesel locomotives to operate regularly on the British main line, although EMD powered locomotives have been the mainstay in both the Republic of Ireland since 1961 and Northern Ireland since 1980.
Split into simpler sentences: They were the first US-built (and privately owned) diesel locomotives to operate regularly on the British main line. However, EMD powered locomotives have been the mainstay in both the Republic of Ireland since 1961 and Northern Ireland since 1980.