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The RMT has said it is appalled that newly refurbished trains will dump human waste directly onto train tracks.

The rail union said the practice, which had ceased after a deal with ScotRail in 2017, would result in its members being "sprayed with human waste".

It comes after it emerged that only one of a new fleet of high-speed trains linking Scotland's cities had been fitted with a human waste tank.

ScotRail described the situation as "far from ideal".

The rail operator said that of the 10 newly-refurbished trains due to come into service, only the train running between Aberdeen and Edinburgh had been fitted with a waste tank.

'Filthy and disgusting'

A deal between the RMT, Transport Scotland and ScotRail had ended the dumping of human waste onto rail tracks in December last year.

RMT general secretary Mick Cash said: "Our members are rightly appalled that ScotRail have sought to rip-up a clear agreement to end the filthy and disgusting practice of dumping human excrement on Scotland's railways.

"This disgraceful and retrograde step must be halted now and the union will take whatever action is required to hold ScotRail to their agreement with the union.

"One minute ScotRail are bragging about their aim to operate the most modern railway in the world and the next they are back to spraying track workers with human waste."

'Behind schedule'

A ScotRail spokesman acknowledged that there was no timetable for the installation of human waste tanks on the trains, which have been refurbished at a cost of £54m.

In a statement, the company said: "We are working flat out to get more upgraded trains into service as quickly as possible, but our supplier, Wabtec, is behind schedule and has let us down.

"To ensure we can deliver services for our customers we will introduce classic high-speed trains until enough upgraded Inter7City trains are available.

"These classic trains will not have retention tanks, which is far from ideal. We are doing everything we can to mitigate this as soon as possible."

A Transport Scotland spokeswoman said it completely sympathised with unions over the issue.

She added: "The Scottish government has directly funded previous installation programmes to eradicate this practice across ScotRail fleets.

"We understand this interim measure is regrettable, but necessary to support more services and jobs across the country while retaining passenger services between our key seven cities.

"Wabtec must deliver their work as soon as possible, so passengers and staff can benefit from these improvements."