Happy new year. Sorry, back to the cable car again.

If you use public transport in London, 2016 opened with a fare rise – and that included the Emirates Air Line, which slapped 10p on an adult single trip. The annual round of fare rises also kicked off campaigning for May’s mayoral election, with the Greens’ Sian Berry demanding London’s fare zones be axed (Woolwich residents fuming at being stuck in zone 4 take note) and Labour’s Sadiq Khan pushing his plan to freeze fares.

Khan told the Evening Standard he would fund the £450m freeze by scrapping Boris Johnson’s “vanity projects”, with our very own cable car in the firing line.

“I’ll start by ending any further public funding for the Emirates cable car as soon as the contract allows — if that means it closes, then so be it,” he said. “It has been a disastrous waste of money and costs more than £5 million a year to run.”

This is cobblers. The cable car’s operating costs are certainly about £5m – but its popularity as a tourist attraction means it’s making a sum quite near that back in fares. This is a bit like Khan demanding the 177 bus is scrapped because it costs £4.6m each year to run – he’s ignoring what it makes back in revenue.

Indeed, I’m indebted to Mayorwatch’s Martin Hoscik for chasing up the figures with TfL – the accounts show it makes a surplus.

An analysis of Transport for London’s audited accounts show that, instead of receiving a “subsidy”, the scheme’s fare revenue met or exceeded operating costs in each of the last three financial years.

In its first nine months of operation, the period covered by TfL’s 2012/13 accounts, just under two million passengers were carried, generating fares revenue of £6m.

During 2013/14 passengers numbers, which were boosted the previous year by the scheme’s novelty and London’s hosting of the Olympics, fell to 1.5 million passengers with fare revenue of £5m.

Passenger numbers remained flat in 2014/15 at 1.5 million but revenue increased from £5 million to £6 million.

We know that operating costs have fallen – which is why a story last summer that the cable car was losing money fell apart. Transport Commissioner Mike Brown’s most recent report to the TfL board said the Emirates Air Line has made a £1m surplus since it opened.

This doesn’t suddenly make the cable car a brilliant idea – user numbers have pretty much flatlined, and Boris Johnson’s stated aim of it paying its build costs (£16m (£60m minus £36m from Emirates and £8m from the EU)) looks like a tricky proposition.

It’s also certainly so far failed as both a commuter link and a generator of significant extra employment, both justifications used for building it.

But Khan was wrong to have highlighted the operating costs.

The big flaw in the cable car is that £16m that could have gone into, say, improving the botched bus infrastructure on the Greenwich Peninsula (where a pedestrian died yesterday morning) has instead gone into a tourist attraction that sits apart from the public transport system with incompatible fares.

Indeed, as Mayorwatch points out, the Emirates sponsorship contract ties TfL into operating it until 2021, by which time London could be on its fourth mayor. There is a break clause in 2017, but it’d be costly for TfL to break the contract and it would lose a big chunk of the Emirates sponsorship cash.

So the mayoral candidates are stuck with a tourist attraction that seems to just about tick over financially. It’s one to watch – maybe tinkering with fares could boost weekday usage – but with TfL losing all its government grant by 2019, there are bigger things to worry about, like protecting bus services.

Asked to comment on the scheme’s published finances, a spokesman for Mr Khan’s campaign said: “We’ll review it… and the likeliest option is it closing in 2021”.

But Sadiq Khan doesn’t know what the Royal Docks and Greenwich Peninsula will be like in 2021, when the Emirates contract ends. Nor do any of us. I’m not sure whether there’ll be a significant commuter traffic (you’d need a lot of people living on the peninsula and working in the Royals, or vice versa) but there’s clear evidence of plenty of leisure traffic. Closing it would leave London one river crossing down – however flawed it may be, and may leave TfL recording an overall loss.

The mayor in 2021 could sell the cable car, which could guarantee an instant return); could cut opening hours (the current contract mandates opening up by 7am, when hardly anybody uses it); or could go the other way and integrate it into the public transport system (which could cost a bit but most public transport costs a bit).

Or the 2021 mayor could carry on as now, with a new sponsor contract, maybe wiping the build costs once and for all, and leave the issue for few years in the future.

The cable car is an intriguing problem for the next mayor – and how they react to it will tell you a lot about them. What we’ve learned about Sadiq Khan is that he needs some new advisors – fast.