The Schweizerische Südostbahn AG (German, lit. 'South-Eastern Railway') – commonly abbreviated to SOB – is a Swiss railway company, and a 1,435 mm (4 ft 8+1⁄2 in) standard gauge network in Central and Eastern Switzerland. It resulted from the merger of the original SOB with the Bodensee–Toggenburg railway (BT) at the end of 2001.

The Schweizerische Südostbahn AG (Swiss South-Eastern Railway SA) is a small private railway jointly owned by the cantonal and federal governments as an Aktiengesellschaft (AG).

The rail network of the Südostbahn (SOB) consists of that formerly owned by BT in northeast Switzerland (between Lake Constance and Toggenburg):

and that previously owned by the original SOB located predominantly in Central Switzerland:

Since 2006, SOB also owns Wattwil station and the railway from Wattwil to Ebnat-Kappel, which until then belonged to SBB CFF FFS. The railway stations of Romanshorn, St. Gallen, and Pfäffikon SZ, and the railway tracks between St. Gallen St. Fiden–St. Gallen, Wattwil (except railway station)–Rapperswil, and Arth-Goldau–Luzern are owned by Swiss Federal Railways, but are used by SOB for its services.

In total, the SOB network measures 128.9 kilometres (80.1 miles), and comprises the following lines:

The adhesion railway network spreads over mountainous terrain, with a maximum slope of 50‰ (5 %) between Wädenswil/Pfäffikon SZ and Biberbrugg, and between Rothenthurm and Arth-Goldau. The lowest altitude on the SOB network is found at Romanshorn (399 metres (1,309 ft) a.s.l.), and the highest at Biberegg (933 metres (3,061 ft) a.s.l.), between Rothenthurm and Sattel-Aegeri. The highest elevation on the section between Nesslau-Neu St. Johann and St. Gallen is reached near Degersheim (798.7 metres (2,620 ft) a.s.l.).

The network is mostly a single-track railway, with intermittent double-tracks (9.84 kilometres (6.11 mi) in total) present in sections where trains cross regularly. The entire network is electrified since 1939.