It is good news that Edinburgh's new trams have carried 1.5 million passengers since they began operating 100 days ago, although there is still a long distance to travel before the worth of the project is proven beyond doubt.

What remains clear is that the planning and construction of the trams was an embarrassing fiasco and there are still many unanswered questions about the roles played by the city council, its arm's-length company Transport Initiative Edinburgh (TIE) and the Scottish Government.

However, credit where credit is due: in carrying 1.5 million passengers, or around 90,000 people a week, the trams have exceeded their targets, although the novelty factor will certainly have helped in the first few days as well as a number of big events such as the One Direction concert.

The drivers of the trams are also reporting that, despite the limited route, commuters appear to be using them every day. In addition, there appears to be no detrimental effect on the city's buses, which may mean more people overall are using public transport in Edinburgh. If the trams have had that effect and encouraged some commuters to leave their cars at home, then that is welcome.

Even so, the flaws at the heart of the trams project remain clear: the cost of the planning and construction rose from £545m to £776m, for example, and the entire project was delayed by three years.

The proposed network for the trams also shrank: in its original form, the trams were to run from the airport to the west, past Haymarket, through the city centre and through Leith to the waterfront at Newhaven; but in the end, the network was truncated and stops at York Place to the end of Queen Street in the city centre.

That was extremely disappointing and if the tramway is to genuinely form an important and useful part of a wider, efficient transport network, the route will have to be extended in the future.

Before that happens, though, there is due to be a judge-led inquiry into the fiasco of the trams project. It is a welcome move, announced by the Scottish Government in June, although the silence since then has been deafening and a timeline for its delivery should be announced as quickly as possible.

As for the questions the inquiry must tackle, they include: how effective was the council's oversight of the project? Could the conflict between the council and TIE have been avoided? And, should the Government have offered more assistance? Lord Hardie's inquiry also needs to explore the question of whether local councils are the right type of body to commission and oversee such huge infrastructure projects.

In the meantime, the people of Edinburgh appear to be taking to their new trams with some enthusiasm, which will help the city move on from the fiasco of their construction. But to make a genuine contribution to Scotland's transport network, the route will have to be extended. The lessons of its construction will also have to be learned if public infrastructure projects are not to repeat the same mistakes in future.