The Tösstal railway line (German: Tösstalbahn or TTB) is a railway in the Swiss canton of Zürich, which serves the communities of the Töss Valley. Passenger service on the line now forms part of the Zürich S-Bahn, branded as the S26, and the standard Zürcher Verkehrsverbund (ZVV) zonal fare tariffs apply to the line. It is one of the network's less-heavily traveled lines, and most of the route is single-tracked.

The Tösstalbahn (TTB) opened between Winterthur Grüze and Bauma was on 4 May 1875, and its continuation to Wald opened on 15 October 1876. At Wald the line connected with the independently owned Wald-Rüti-Bahn (WR) from Rüti ZH, which had itself opened on 29 September 1876. The two lines remained in separate ownership until they both became part of the Swiss Federal Railways (SBB) on 10 June 1918, although the TTB had operated the WB from 1902.

In 1901, the Uerikon-Bauma-Bahn (UeBB) opened, providing a third access point to the Tösstal line at Bauma. This line was never a great success, and in 1948 much of it was abandoned, but the stretch from Hinwil to Bauma was acquired by the SBB, retained and electrified. However passenger services ceased by 1979, and the Hinwil to Bauma section is now operated as a preserved railway by the Dampfbahn-Verein Zürcher Oberland (DVZO).

In 1990, the passenger service on the Tösstalbahn became part of the Zürich S-Bahn network. Operation remained the responsibility of the SBB, with service provided using RBDe 560, with occasional RBe 540 supplements. Einheitswagen ("standard coaches") I or II as well as a BDt or Bt control car made up the rest of the consist, all of which were generally in the NPZ (blue over white) livery.

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