Complex sentence: In 1947, when Pakistan achieved its independence from Britain, the North Western State Railway was renamed to Pakistan Western Railway while in East Bengal, the portions of the Assam Bengal Railway was renamed to Pakistan Eastern Railway.
Split into simpler sentences: In 1947, when Pakistan achieved its independence from Britain, the majority of the North Western State Railway infrastructure fell under Pakistan's territory and was renamed to Pakistan Western Railway. In East Bengal, portions of the Assam Bengal Railway fell under Pakistan's territory and was thus renamed to Pakistan Eastern Railway.