Chas Allen and Bruce Lake are quite right about the initial impact of modern trolleybuses (and even diesel buses) replacing some pretty ancient “boneshaker” trams but any gain in passengers was rapidly lost after WWII as private motoring took hold and the acclaimed flexibility of the rubber-tyred modes simply rendered them victims of the same traffic jams the cars (and vans and lorries) got snarled up in.
In countries like Germany, Austria, Switzerland, Belgium and The Netherlands, post-war public transport use held up much better than in Britain as trams had been retained and modernised, and the proportion of reserved tracks grew substantially. That was set to happen in at least Glasgow, Leeds and Liverpool (where many miles of reserved track had been installed) but the rapid Americanisation of British life (and politics) led to the hugely wasteful abandonment of those three systems and every other tramway bar Blackpool. If only we had been more European in the 1950s – and now!
The Leeds trolleybus project failed (not least because the Public Inquiry Inspector hadn’t the first idea what a trolleybus is) but its capital costs were only part of the story. Whole-life costs for the predecessor Leeds tram project would have been lower, as is the case with most tramways, but the UK Treasury only knows the price of a bus and not the value of a tram (and it thinks two weeks is long-term)!
The lower capital cost of trolleybuses was not, in fact, the real reason for them replacing trams but rather the need to amortise the fixed investment in substations, etc. in the accounts of the municipalities concerned. Significant capital was still expended in the installation of trolleybus overhead, of course, and it is interesting that in London the new LPTB would have preferred to replace the trams with diesel buses but the accountants pointed out the waste of assets that were not life-expired.