Le Day Viaduct, Railway bridge in Vallorbe, Switzerland.

Le Day Viaduct is a railway bridge over the Orbe near Vallorbe, rebuilt in stone between 1923 and 1925 on the piers of an 1869 iron truss bridge. Its main arch has a clear span of 44 m, with nine small arches of 4.50 m above it, and the deck is 8.4 m wide. A 1.5 m footpath runs beneath the arches.

The viaduct was built in 1870 by the Jougne-Eclépens Railway as a connection between regions. Between 1923 and 1925, it underwent major reconstruction in which the original iron framework was replaced with stone arches to handle heavier trains and electrical systems.

The viaduct forms part of the rail connection between Paris and Lausanne, linking Switzerland to France through TGV Lyria services.

A 1.5-metre-wide pedestrian passage runs under the bridge vaults, linking the cantonal road to the Vallorbe-Le Creux customs post with Le Day. On the left bank, a path follows the Orbe past the reservoir of the Day dam, whose water now surrounds the bases of the two large piers.

The 1923–1925 rebuild kept trains running: the arches were first erected inside the existing openings beneath the iron girders, with timber centring by Richard Coray, then the old superstructure was demolished section by section. The piers and abutments of the 1869 viaduct were reused, and the stone came from the Arvel quarries near Villeneuve.