As Peter has said the 'best' trains got E cars and the rest got Ws if they were lucky plus a sprinkling of PLs.
As a generalisation, the W cars would have run everywhere on just about anything and everything except the Spirit and the Overland at one stage or other of their life.
While technically true that they could have run anywhere, and on special trains probably did, in practice they'd only be seen on a relatively small number of lines that ran locomotive hauled passenger trains. I can't imagine that an AW was commonly seen at Kooloonong.
Almost all branch lines and even secondary main lines were served by railmotors/railcars and would have rarely seen any 'modern' passenger car. At best the secondary main lines would have seen a locomotive hauled RM replacement train during peak travel times (e.g. Easter) made up of 19th C stock. In the brief period between the introduction of the W class and the widespread replacement of mixed trains by railmotors, the stock used on the mixed trains would have been the aforementioned 19th C stock, not the brand new quality stock.
Lines on which loco hauled passenger trains ran (post introduction of the W class) included: Melbourne - Geelong - Port Fairy; Melbourne - Ballarat - Ararat - Dimboola (and probably Adelaide occasionally on 2nd division Overlands); Melbourne - Mildura; Melbourne - Bendigo - Swan Hill; Melbourne - Shepparton - Tocumwal; Melbourne - Albury; Melbourne - Bairnsdale; Melbourne - Foster; Melbourne - Crib Point - Stony Point; Melbourne - Healesville & Melbourne - Warburton. If you add Sunday excursions, you can add Daylesford and Mornington.
However, the W class cars were only one class of 'modern' cars and wouldn't have been found on all of these lines. In the mid '60s, for example, the WTTs suggest that W class cars were mainly used on the shorter & medium distance country services - Geelong, Ballarat, Seymour etc. Longer distance services were the domain of the better cars - E, S, & Z, although ABWs & CWs were used for strengthening on heavy days.