Mostly filmed in summer 1994 this film primarily shows trolleybuses hauling trailers, although also seen are some trolleybuses without trailers, plus a few scenes showing the TSOL light rail and the former cog wheel / rack & pinion line which has now been replaced with an automated mini-metro that uses the same trains as the fully automated driverless lines on the Paris metro.
The reason for using trailers is that they offer a cost effective way to increase passenger capacity when the fleet of buses (which are not articulated) still have many years of life left in them. At quiet times it is still possible to operate the bus on its own.
This film shows two different generations of trailer, the most reason trailers are of the 'low floor / easy access' variety. Introducing trailers with this configuration into their fleet made it possible to offer enhanced accessibility without having to prematurely scrap the existing fleet of high-floor trolleybuses when they still had many years of revenue-earning life left. However that was in the early 1990's and I saw it said somewhere that in circa 2018 the trolleybuses which use trailers (and their trailers!) will be replaced with the latest design of double-articlulated (bi-articulated) trolleybuses.