Mälardalstrafik
| A Mälartåg ER1 Train | |
| 
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| Company type | Aktiebolag | 
| Industry | Transport | 
| Predecessor | Tåg i Mälardalen AB | 
| Founded | 1991 | 
| Headquarters | Stockholm, Sweden | 
| Area served | Stockholm Mälaren Region | 
| Revenue | 1.3 billion SEK (2023) | 
| Total assets | 392 million SEK (2023) | 
| Owner | 
 | 
| Number of employees | 
 | 
| Website | https://malardalstrafik.se/en/ (Corporate) https://malartag.se/en/ (Train Services) | 
| Mälartåg | |||
|---|---|---|---|
| Logo of Mälartåg | |||
| Overview | |||
| Number of lines | 5 | ||
| Number of stations | 44 | ||
| Annual ridership | 12.5 million (2023) | ||
| Operation | |||
| Operator(s) | Transdev | ||
| Number of vehicles | 59 | ||
| Technical | |||
| System length | 102.3 km (63.6 mi) | ||
| Track gauge | 1,435 mm (4 ft 8+1⁄2 in) standard gauge | ||
| Electrification | 15 kV 16+2⁄3 Hz AC from overhead catenary | ||
| 
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Mälartåg is a Swedish regional train operator serving the Stockholm-Mälaren Region. It is managed by Mälardalstrafik, a company jointly owned by the regional public transport authorities in the Mälardalen area, including in Stockholm, Sörmland, Uppsala County, Västmanland, Örebro County, and Östergötland. The name Mälartåg means Mälaren-train in Swedish while Mälardalstrafik means Mälaren Valley transport.
Mälardalstrafik is responsible for managing and developing regional train services in these areas. Along with operating Mälartåg, the company offers Movingo, a period ticket option for commuters. Mälartåg primarily uses Stadler ER1 electric trainsets, with some routes also featuring Bombardier Regina X50 trains. Since June 2024 Mälartåg services have been run under contract by Transdev.