The Weilburg Ship Tunnel was bored through the sandstone ridge beneath the old town in the 1840s to allow boats on the Lahn to bypass the tight river loop around which the town is built. Germany's only ship tunnel, it runs just over 200 metres through the rock and saved river traffic from a considerable detour around the meander. It is still navigable today for small pleasure craft and canoes, and the experience of paddling through it is genuinely memorable.
A pedestrian walkway alongside the water channel allows non-paddlers to walk through the tunnel. The tunnel entrance on the downstream side is clearly signposted from the town centre and is about a ten-minute walk from the Schloss. The walk through the tunnel takes around five minutes.
What makes the site particularly unusual is that a railway tunnel and a road tunnel run in parallel through the same hillside — three separate tunnels for three separate modes of transport, side by side, which is believed to be unique worldwide. The ensemble gives Weilburg an oddly singular place in the history of 19th-century civil engineering.