Nearly a dozen Queensland Rail train drivers and guards have been caught under the influence of alcohol or drugs at work in the last two years.
QR statistics from the last five years show an increase in the number of positive results from random drug and alcohol tests.
One person was caught in the 2010/11 and 2011/12 financial years, however that increased to four in 2012/13, five in 2013/14 and six in the current financial year.
Those who fell foul of the QR zero-tolerance policy only make up a fraction of the staff who work as guards or drivers. A QR spokeswoman said there had been no injuries as a result of actions by workers who tested positive.
QR chief operating officer Kevin Wright said the authority had increased the number of random tests it performed in the last 18 months.
"Following this we have seen an increase in the number of drivers and guards testing positive," he said.
"Any person testing positive is immediately excluded from the workplace and enters a disciplinary process.
"None of the drivers testing positive in these last eighteen months has returned to work and all have been dismissed, or are still finalising their disciplinary process."
Mr Wright said QR had hired an independent drug and alcohol expert to review existing policies.
Rail, Tram and Bus Union state secretary Owen Doogan said the problem of impaired drivers was rare.
"Sometimes it's simply because they were out the day before and thought they had sufficient time to get it through the system," he said.
"There are a few thousand drivers and guards. [The positive tests] is a low figure and most of them would be able to drive a car because they'd be under the .05 limit."
While Mr Doogan said he supported increased random testing, he believed there had been occasions when dismissals were overly harsh for staff with very low readings.
Queensland's Transport Minister Jackie Trad said her priority was passenger safety.
"Everyone has the right to feel confident when travelling on the rail network and people should be assured of the safety of Queensland Rail's network," she said.
"This is concerning and unacceptable behaviour and will not be tolerated."
