That's not entirely what public transport companies have found. Busses don't scale as well as using entire trains. I have seen a bus route replaced by a tram, because they had to send double-length busses at every few minutes. Which in practice meant that you would see two or three of those busses arrive at the same time. A well-planned public transport system will use a mix of busses of diverse sizes in conjunction with trains/trams for trunk routes.