Deiseler Tunnel, Railway tunnel near Deisel, Germany

The Deiseler Tunnel is a 202-metre railway tunnel through the Kesselberg near Deisel, built 1846–47 for the Carlsbahn. It is slightly curved, 6.05 m high and 4.60 m wide, with sandstone abutments and brick lining. The north portal is brick with pilasters, a frieze and a crowning parapet; the south portal is faced with sandstone.

The Deiseler Tunnel was built from 1846 to 1847 for the Carlsbahn by the Kassel firm Conrad Siebrecht; the first concrete work began on 23 April 1846, the brick vault was lined from August 1846, and the tunnel was finished on 1 June 1847. The line opened in 1848 and the tunnel was closed to rail traffic in 1986. Its builder's memorial plaque gives February 1846 to June 1847 as the construction period.

The tunnel and its companion building are protected as cultural monuments under Hessian law, preserving historical architectural features from the 1840s. Visitors can observe the craftsmanship of that era in the decorated sandstone portals and ornamental details.

The tunnel is open to cyclists and hikers from 1 April to 31 October; from 1 November to 31 March it is closed as a hibernation site for bats. The cycle and hiking routes R4, Diemelradweg, Märchenlandweg and Eco Pfad Diemel pass through it. South of the south portal, the former railway embankment in the Holzapetal nature reserve must still be bypassed on foot or by bike for a few hundred metres.

The excavation needed no temporary support structures during construction because the rock was exceptionally hard. This allowed workers to line the passage directly with brick without complex scaffolding.