Melbourne was unusual amongst Australian cities in that it had a large number of independent tramway operators . All other Australian cities had tramway monopolies, at least in the electric tramway era.
Therefore in Melbourne the construction of electric tramways was driven purely by local municipal concerns, with no central planning or coordination. Each individual tramway organisation ordered small batches of electric tramcars from the lowest bidding independent car builder  using a variety of designs. This led to Melbourne having an eclectic mix of different electric tramcar types.
However, unlike the electric tram operators, the cable tram operators used highly standardised designs, with only two types of dummy cars and four types of trailer cars, totalling almost 1200 cars. This was due to the dominant position of the Melbourne Tramway & Omnibus Company (MTOC) and its successor, the Melbourne Tramways Board. It was more cost effective for the operators of the independent Northcote line to purchase standard rolling stock from MTOC than to design and build its own.
A typical example of a standard Melbourne cable tram set is on display in the Melbourne Tram Museum @ Hawthorn Depot.
When the Melbourne & Metropolitan Tramways Board (M&MTB) took over the independent operators between 1920 and 1922, with the splendid exception of the Victorian Railways (VR) lines, it inherited 216 electric trams of twenty-one different types, which it proceeded to classify into one of the alphabetical classes A-H or J-V. Through the vagaries of fate, with some assistance from tramway enthusiasts, many of these varying tramcar types survived into the current day, and we are fortunate to have some of them on display in the Melbourne Tram Museum @ Hawthorn Depot.
The M&MTB decided not to continue the practice of its predecessors in purchasing small batches of tramcars of different designs from a variety of car builders. Instead, it chose to become a vertically integrated tramway operator, capable of designing, building and maintaining its own tram fleet . It built its own major tramway workshops on vacant land at Preston, on the corner of St Georges Road and Miller Street. Preston Workshops, as it became known, had all the facilities to build new tramcars as well as maintain them. After a little experimentation, the M&MTB decided on a standard design that it called the W class tram, and from 1923 to 1956 it proceeded to build twelve different major variations and over 750 examples of this basic theme.
The M&MTB also experimented with the construction of a small number of trams of different designs between 1924 and 1930 before it standardised on the W class. These variations were placed into one of the subcategories of the X and Y class trams.
In 1973 after a long hiatus in new tramcar construction, the M&MTB built a prototype tramcar, No 1041, subsequently used as the basis of the new Z class design, which revolutionised the appearance of Melbournes tramways from their introduction in 1975.
Examples of these historic electric tramcars built for the M&MTB are on display in the Melbourne Tram Museum @ Hawthorn Depot.
Cross, N., Budd, D., and Wilson, R. (1993) Destination City (Fifth Edition), Transit Australia Publishing
Cross, N., Henderson, R. and Kings, K. (1981) Destination City (Fourth Edition), Australian Electric Traction Association
Hawthorn Tramways Trust (1916-19) Annual Reports
Jones, R. (2008) Fares Please! An economic history of the Melbourne & Metropolitan Tramways Board, Friends of Hawthorn Tram Depot
Keating, J. D. (1970) Mind the Curve! Melbourne University Press
Macmeikan, I. (1956) Melbourne Cable Tramways, Melbourne & Metropolitan Tramways Board
Melbourne, Brunswick & Coburg Tramways Trust (1916-19) Annual Reports
Melbourne & Metropolitan Tramways Board (1920-82) Annual Reports
Prahran & Malvern Tramways Trust (1910-19) Annual Reports
Melbourne cable tram operators prior to formation of M&MTB:
Melbourne horse tram operators prior to formation of M&MTB: