This CNR roundhouse was near the present-day Queensway's Nicholas Street turnoff. Source.
This CPR roundhouse was at Nepean Bay just to the west of Lebreton Flats. Source.
The remains of this turntable built in 1871 for the St. Lawrence and Ottawa Railway were uncovered just a little to the west of the CPR one. Source.
The Canadian Northern Railway had a roundhouse at 300 West Hunt Club Road. Source.
The Canadian Atlantic Railway had a roundhouse on Elgin Street. Source.