The following is a response to a research inquiry:
Stratford Beacon - May 2, 1873 - mentioned in article; "The location of the Great Western repair shops for the W.G.B.R.R. here, will through the hands employed considerably increase the size and population of the village. The railway company are now building a large engine shed and have completed a tank and turn table.
Ian Wilson described the roundhouse as follows:
Originally built in 1872, the Palmerston roundhouse is surrounded by a variety of buildings. In addition to the office, there is a classroom built in the 1940s, replacing a boxcar which had served the same purpose. A bunkhouse was also added in the previous decade. The third stall from the north is home to Betsy, the boiler which heats all the important railway buildings, including the station, Queen's Hotel and the car department. Coal for Betsy is stockpiled by hand in a shelter built two years ago, located next to a track which runs through the fourth stall. Off the north edge of the roundhouse, a red metal-clad building contains a lunch room and storage space for firebrick, waste, oil and other materials. Behind the roundhouse, off the southernmost of the two storage sidings, formerly stood the 40,000 cu. ft. ice house, built in 1906 and stocked by carloads from Capreol, Allandale and Stratford.
November 10, 1876 - Listowel Banner: The station was expanded to a 60' x 30' rectangle. A roundhouse and roundtable, coal sheds, cattle pens, and freight shed were added at this time.
Important Events that likely affected the roundhouse:
1882 - merger of the Great Western Railway and the Grand Trunk; the merger determined the closure of the small engine terminal in Listowel and moved to Palmerston because of the larger yard and equipment shops. This would have required an expansion of the roundhouse.
1900 - a new 50'6" turntable is built to accommodate the larger engines being constructed. This likely prompted the expansion of the roundhouse as well. A photograph in Peter Bowers book "Two Divisions to Bluewater" shows engine #2485 in 1900 on the turntable with the roundhouse showing similar construction to the building in 1950, however there are notable differences. *see photograph "roundtable construction"
1904 - the Grand Trunk announces it is doubling the size of the Palmerston railyard to accommodate the rapid increase in traffic. Somewhere around this time the turn table is expanded to 70' long. It is likely the roundhouse was expanded or reconstructed at that time in its final form. *this was also the year the arena was built on William St.
1959 - Closure and tear down of Roundhouse
*I've noticed in all the articles from the centennial paper the roundhouse crews are all spoken highly of, an essence of hard working but fun loving characters who were the reason why the railway ran smoothly.