The Dublin-Cork train line has been labelled a "drug trafficking" corridor by the National Bus and Rail Union (NBRU).
The union has repeated its call for a dedicated transport police unit to tackle anti-social behaviour.
It comes as former government minister Willie O'Dea suggested parents of children who cause trouble on public transport should face sanctions.
General Secretary of the NBRU Dermot O'Leary says the problem is widespread on transport routes.
"Anecdotal evidence we have from our own frontline staff members, for example, on the Cork-Dublin line, or people telling us, that it's now become a drug trafficking corridor at this stage," he said.
A lot of people who have been engaging in this type of activity would be engaging in anti-social behaviour on trains.
"You have a situation on the Dublin-Sligo line that's a problem, along with several parts of that line," he said.
He said there was a situation in the west of the country whereby one of the train drivers was seriously assaulted last year.
Irish Rail last week announced plans to introduce a text alert system for passengers along with the expansion of CCTV to all carriages.
A company spokesperson said that this year will also see the doubling of security patrols.