Quebec, Montreal & Southern Railway Reporting mark: DH

The Quebec, Montreal & Southern Railway (QMS&R) was an American-owned line that operated from 1906 to 1929.

The QMS&R began in 1890 as the East Richelieu Valley Railway (ERVR). It was chartered to build a line from Saint Hyacinthe to Lacolle. In 1895 the ERVR was purchased by the United Counties Railway which had recently opened lines from Iberville to Saint Hyacinthe and Saint Hyacinthe to Bellevue Junction.

Although the acquisition was mutually beneficial, United Counties continued operating the ERVR as a separate entity. In 1898 the ERVR expanded from Iberville to Noyan. In 1900, it was purchased by the Rutland Railroad and renamed the Quebec Southern Railway (QSR). The United Counties Railway was then merged into the QSR. In 1902 a new section was opened from Saint-François-du-Lac to Pierreville.

In 1894 a second railway, the South Shore Railway (SSR) was beginning to take shape The SSR was originally created to build a line from Valleyfield to Lévis and it grew quickly. By 1900 it had acquired the Montreal & Sorel Railway Railway, the Great Eastern Railway and the Montreal & Atlantic Railway line that ran from Sorel to Yamaska Est.

The Delaware and Hudson Railroad (D&HRR), eager to expand in Quebec had its eye on both the QSR and SSR. Beginning in 1903 the D&HRR began purchasing interests in both railways. In 1905 DH took full ownership of the SSR, followed by the QSR in 1906. The two railways were then merged to form the Quebec, Montreal & Southern Railway (QM&SR), which was operated as a subsidiary of D&HRR. Further extensions under D&HRR ownership included Pierreville to Nicolet and the Saint-Grégoire Junction to Fortierville.

The QM&SR remained under DHRR ownership until 1929 when it was purchased by the Canadian National Railway (CN). Although numerous sections of the line have since been abandoned, CN still retains ownership of the railway. Following bankruptcy in 1990 D&HRR was put up for sale and in 1991 was purchased by the Canadian Pacific Railway (CPR). It still remains under CPR ownership.