What is "seedamm"

Seedamm is the partially artificial causeway and bridge at the most narrow area of Lake Zurich, between Hurden (SZ) and Rapperswil (SG). The Seedamm carries a road and a railway across the lake, with the railway being used by the S5 and S40 lines of the S-Bahn Zürich and by the Südostbahn Voralpen Express.