Translink has been urged to drop a ban on reusable coffee cup refills on board cross-border rail services.
The public transport provider offers its own reusable cup for travellers which works with its drinks systems but refuses to refill other cups over safety concerns.
Irish Rail - which jointly operates the Belfast to Dublin service with the Northern Ireland rail operator - also enforces the policy.
Irish Rail said there were concerns over scalding. It said the health and safety of staff and customers was a "top priority".
SDLP Councillor Gary McKeown called on Translink to drop the ban, saying it will result in "needless waste and inconvenience for travellers".
"At a time when local and high street coffee shops are offering discounts for reusing cups, it is unfortunately against Translink policy for passengers to receive hot drinks in their own reusable cup."
A Translink spokesperson said: "Our Enterprise Keep-Cup has been carefully designed to be compatible with on-board catering trolley equipment offering a quick and convenient service while avoiding the risk of spillage or scalding either staff or customers.
"Therefore, only this specific Enterprise Keep-Cup is accepted on board Enterprise and any other local rail services with on-board catering services.
"Customers can buy their Enterprise Keep-Cup on board Enterprise services for just £2.50/€3.00 and this price also covers the cost of your first hot drink purchase.
"In addition, we have a loyalty reward scheme where customers who then re-use their Enterprise Keep-Cup will enjoy 10% discount on hot drinks from our on-board trolley service."
Councillor McKeown said if Translink was offering a free drink with its own cup there would be little incentive for customers to keep it and they could end up binning the reusable cup.
"I understand that health and safety is part of the issue, as the catering trolleys cannot easily accommodate cups of different sizes," he added.
"This means that it is essential for Translink to look at its equipment to make sure it is fit for purpose to help protect our environment, while ensuring the safety of staff and passengers.
"Public transport helps cut carbon emissions by reducing reliance on cars, but it can't just stop there – we all have a responsibility to take steps towards a greener society by reducing waste as well, and I urge businesses to think creatively about how they can support this.
"We can't afford the luxury of a 'computer says no' response to environmental challenges."
A spokeswoman for Irish Rail told the BBC their own "bespoke" cup "is designed to fit under the spout of the trolley and the lids have been tested for safety purposes".
She said cups brought on to the train often do not fit under the nozzle of the hot drinks machine and also cited train speeds as a concern.