The Bremgarten–Dietikon railway line (German: Bremgarten-Dietikon-Bahn, BD) is a railway line in Switzerland. It connects Wohlen and Bremgarten in the canton of Aargau with Dietikon in the canton of Zürich. Passenger service on the line now forms part of the Zürich S-Bahn, branded as the S17, and a frequent service is provided, with trains running up to every 15 minutes.

The line was opened in stages, and under different ownership, between 1876 and 1912. Whilst the earliest section to be opened, between Wohlen and Bremgarten, was built as a traditional standard gauge branch line, the later section between Bremgarten and Dietikon was built as rural electric tramway in metre gauge. Whilst much of the line has been upgraded, this is still apparent in the long stretches of roadside track, and stretches of street running in Dietikon.

Today the line is owned and operated by BDWM Transport AG, a company that also operates bus services in the region.

The route between Wohlen and Bremgarten West was built to standard gauge by the Aargauische Südbahn railway, which connected Aarau and Rotkreuz via Wohlen. The service began on September 1, 1876, operated by steam locomotives.

On January 1, 1902, the Swiss Federal Railways (SBB) took over the Aargauische Südbahn. The SBB continued to operate the service as a steam operated standard gauge branch line.

The line to Wohlen had several disadvantages for Bremgarten. The timetable was poor, Bremgarten West station was too far away from the town centre, and the connection to Zürich initially proceeded in the wrong direction and was very indirect. In 1895 a plan was produced for a roadside electric tramway between Bremgarten and Dietikon. As Dietikon already had a direct rail link to Zürich, this provided a much more direct connection.

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