Complex sentence: A little north of the Roertunnel, it's sistertunnel the Swalmertunnel has been constructed according to the same principals, this tunnel measures 400m in length and leads underneath the urban area of Swalmen, a municipality of Roermond.
Split into simpler sentences: A little north of the Roertunnel, a sister tunnel, The Swalmertunnel, has been constructed according to the same construction principals. That tunnel measures 400m in length and leads underneath the urban area of Swalmen, a municipality of Roermond.