Complex sentence: Marketed as the Cathedrals Express (Ely, Lincoln and York being the three cathedrals) the train used the Great Northern and Great Eastern Joint line between March and Doncaster.
Split into simpler sentences: Marketed as the Cathedrals Express (Ely, Lincoln and York) being the three cathedrals. The train used the Great Northern and Great Eastern Joint line between March and Doncaster.