Edinburgh’s £776 million tram line is running at an average of 25 per cent capacity, figures published today by the Scottish Conservatives showed.
Passenger numbers almost doubled between 2014 and 2015 to 5.3 million, but the services have the potential to carry 21 million a year.
Each tram has a capacity of 250 and the vehicles make a total of 85,000 journeys per year - meaning on average three-quarters of the space on the network is unfilled.
A public inquiry has been set up to investigate the huge problems which dogged the building project, which went massively over budget and was completed in May 2014 - five years later than planned, having caused major disruption for residents and visitors through years of works.
Lothian Conservative MSP Miles Briggs said: “The one saving grace people in Edinburgh thought they could take from the trams was they would be worth the wait eventually.
“However, with three-quarters of spaces unfilled every year, it seems even that consolation hasn’t come to pass.
“This was a hideously disruptive and expensive project but, having been operational for a while, millions of spaces on these trams are going spare.
“Passengers are complaining they continue to be slower than the bus, and some residents are so furious with the inconvenience they caused they refuse to get on them.
“It’s clear there is still significant work to do to make this project a success, and I hope bosses use these numbers as a catalyst for doing exactly that.”
The figures were revealed by Edinburgh Trams finance manager Tom Neil in response to a freedom of information request, which said during peak times some trams “operate at 100% capacity” and enhanced provision had been made to cope with demand.
An Edinburgh Trams spokesman said: “It’s disappointing the Conservatives have issued a figure that misrepresents both the success of the trams and how public transport in general operates.
“We’ve already had to introduce extra services at peak times and there will be good news early in the new year on further service improvements.
“Although a relatively recent form of transport for the city, the tram continues to go from strength to strength and is clearly proving its value for the people and economy of Edinburgh.”
The firm said the figures were based on 2015 passenger levels and it expects 2016 to show a significant increase.
It said two-thirds of a capacity of a tram is standing, and based on available seats, usage levels are around 80 per cent.
Bus Users Scotland director Gavin Booth said: “This is an unusual way of looking at the information that has been obtained.
“All operators of public transport know there are times when buses, trams or trains will be very full and others when they will be much emptier.
“If they only operated at these busy times, passengers whose journeys were less time-sensitive would lose out, and the whole transport offer would be much less attractive.
“Edinburgh Trams runs a service from 5am, when passenger numbers will be low, right through the day, with extra trams at peak times to meet demand, to the last tram arriving at Gyle Centre at 11:54pm.
“What is important is that passenger numbers are increasing year on year, and there is great potential for growth if and when the system is extended.”