Once more that phantom chariot the Bob Crow-crushing "driverless" London Underground train has been summoned by Mayor Johnson in the hope of drawing hearty cheers of "good old Boris!" from strike-maddened commuters - and, of course, sticking a bit of red scare dirt-by-association to his Labour challenger Ken Livingstone in the process.

The last time Boris's "driverless" train set came out of its box was just over a year ago, when the headlines claimed that these saviour vehicles would "break" the RMT union. It was twaddle then and it is twaddle now. Let's recap…

London Underground already has trains that are "driverless" in the sense that the term "train driver" is traditionally meant. The Victoria, Central and Jubilee lines are operated by Automatic Train Operation (ATO) systems - the Victoria partially so since 1968 - which mean they aren't manually controlled by people sitting in cabs at the front end except in an emergency. The main responsibility of those individuals is to safely operate the carriage doors. Also, they are members of trade unions.

Trains on the Docklands Light Railway (DLR) don't have drivers even in the ATO kind of way. Instead, they have "train attendants" or "captains" who travel on the train but move around inside it rather than sitting at the front. These people do, however, look after the doors just like their ATO Tube counterparts. They too are expected to operate the train manually if something goes wrong with the system. And they too are members of trade unions. Indeed, Crow's RMT lays claim to nearly all of them.

So if staff on the truly "driverless" DLR are every bit as unionised as those on the Tube – and prepared to withdraw their labour too - how does Boris think making the Tube progressively more driverless will weaken the Tube unions?

The Evening Standard's report, flammed up in a style reminiscent of its wretched Wadley era, described the long-gestated plan to introduce ATO to the Northern Line in two years time and others in 2018 as enabling Boris to "take away the unions' ability to hold the capital to ransom," but failed to explain why.

The accompanying leader column at least tried. "Automation would mean that any qualified member of staff could act as train captain, diminishing the disruption caused by strikes," it said. But how many London Underground staff doing different jobs would also have been trained to be stand-in Tube train "captains" – let alone be willing to cross picket lines – in order to keep the system running?

Transport for London tells me that around 1,950 Tube drivers are needed to staff the system on an average working day. Looks like an awful lot of double-trained strike-breakers would be required to cover for those withdrawing their labour. In short, the "driverless Tube" pitch is, as Londonist puts it, one of those transport promises you shouldn't trust.

As for making it as a means of mentioning Ken Livingstone's good relationship with ASLEF and the TSSA - though not the RMT – in the hope of winning votes, perhaps Boris should think again. There have been many more Tube strikes under him than when Ken was mayor. People might start thinking that's not a bad reason for making Ken mayor again.