Are Vlocities designed with non powered trailers in mind though? My thinking would be that any non powered trailers would effect both acceleration and top speed meaning the Vlocities wouldn't be capable of keeping to their timetable. I am no expert, but this I beleive is the reason for all cars being powered.
As for 4 car and 3 car sets,I am under the impression the plan is too have all sets made up with 3 cars, meaning they are run as either 3 or 6 car trains. I also believe the planning is already in place to run 9 car sets at peak times when it is no longer possible to run 7 car sets.
A four car V/Locity with 3 powered cars and one true trailer quite well powered for Geelong & Traralgon lines where 4 car sets would be used . Current VLP mentality of must have all 3 car sets is very expensive both acquisition and operation wise . With 3 car sets one can only run as 3 or 6 or 9 cars . This results in many trips running with gross overcapacity . As operating and maintenance costs are distance based running a 6 car VL when 4 cars would be enough adds up to a lot of wasted $ over a year .
So real need of VLP is for more seats, more cars, not additional train sets . VLP needs 24 - 30 true trailers to be inserted into 24 - 30 of the 3 car sets . One then uses the 4 car sets on Off Peak services and as 8 cars at Peak periods .
With RRL and 20 minute Geelong Off Peak tt VLP will end up running 6 car VL sets as 3 car VL will not be enough with extra stations. But with 4 car sets that will be OK Off peak.
PS: There are no platforms beyond Wyndamvale suitable for VL consists over 7 cars. 8 car VL sets would fit at stations Southern Cross, Footscray, Sunshine, Tarneit, Wyndamvale, North Shore & North Geelong . Platforms can be extended to accommodate 8 cars easily at Lara, Corio, Geelong, and Waurn Ponds. To operate 8 car consists at Little River, South Geelong and Marshall presents problems with track and signalling .