Plans for a new tunnel-cleaning train to hoover up potentially harmful dust on the London Underground network have hit the buffers after the project ran into technical difficulties, E&T Magazine has learnt.

The scheme was first mooted six years ago when then Mayor of London Boris Johnson said Transport for London (TfL) had commissioned a special type of train to clean tracks during what are known as engineering hours when no passenger services run.

Two years later Johnson announced the train’s introduction was being delayed by four years because of issues to do with the removal of material on the Tube network which had been found to contain asbestos.

Now it has emerged the project has been quietly ditched – a development that is likely to prompt questions about how much money was spent on it as well as concern from air-quality campaigners who had called for it to be rolled out speedily. A TfL spokeswoman this week confirmed the dust-busting solution had been found to be not viable.

Cleaning of tube tunnels and track continues to be done manually by workers, traditionally known, colloquially at least, as ‘fluffers’.

The only available information about the commissioning process for the vacuuming train comes from an article in the trade press stating it was supplied to TfL by German company Schoerling Kommunal.

Railway insiders say vehicles produced by this company would have had to be significantly altered for use in the Underground, however, because of differences in the track gauge.

Dust on the Underground system has been found to contain small amounts of some metals as well as traces of quartz and, of course, dead skin from the billion passengers who use the network each year.

The dust is mostly composed of iron as a result of grating of the train wheels on the steel track.

A recent study by an academic at the University of Surrey found commuters on the Underground were exposed to relatively high quantities of large-sized particulate matter which Clean Air in London claims represents a health risk.

In 2013, when it was announced the tunnel-cleaning train project was being put back, the campaign group said it was “very concerned” and demanded the scheme be expedited to reduce dust levels.

TfL insists levels of particulate matter on the Tube are perfectly safe.

Dr Olivia Carlton, TfL’s head of occupational health, said: “Passengers and staff can be assured that we have been monitoring levels of dust on the Tube for many years and they remain well below the limit set by the Health and Safety Executive.”

As well as the health issues, dust is a concern because of fire safety.

Bridget Fox from the Campaign for Better Transport described the University of Surrey study into air pollution on the Tube as “a bit of an outlier report”, but she added: “That’s not to say it isn’t an issue.”

She said the problem was “probably being addressed” already through a programme to introduce better ventilation in confined spaces on the Victorian-era network.