Complex sentence: Both buildings are to a design in the Italianate style by Brunel for the Great Western Railway, and are the only intact survivors of this, once common, design, although a much modified example exists at Chepstow railway station.
Split into simpler sentences: Both buildings are to a design in the Italianate style by Brunel for the Great Western Railway. They are the only substantially intact survivors of this, once common, design, although a much modified example exists at Chepstow railway station.