Irish Rail received reports of the use of cocaine, ketamine and other drugs on trains last year.
They're among 277 anti-social behaviour complaints the company received in 2021 - up from 258 the previous year.
On the Belfast train, it's claimed a woman was 'intimidated' and 'preyed' upon by a group of drunk men.
Dermot O'Leary, the general secretary of the National Bus and Rail Union, says dedicated transport police is needed.
"The Gardai are the best equipped in this country to police [transport], we've always said that and that's the route we've been pursing ourselves. The Garda representative bodies themselves have been very supportive of that.
"Obviously they would need further resources from Government to do that, but certainly it would need a proper policing resource, to have the power of arrest."
