The company that runs the Luas in Dublin is being called on to introduce longer trams and more frequent services.
It comes after a tram broke down on the Green Line for a second day running on Wednesday, which led to major delays.
Fine Gael Senator Neale Richmond said users experienced "lengthy delays" trying to get on trams, and were left standing on platforms.
He said: "It seems now that lengthy delays have simply become the norm for Luas commuters as trams are increasingly overcrowded and frequency is becoming ever more sporadic.
"Understandably vexed commuters once again inundated my office all day yesterday and again this morning with complaints following yet more terrible service.
"We are encouraging people not to use cars yet many who would ordinarily use the Luas Green Line are forced to taxis to avoid being late for work or walk.
"In the mornings this week alone, delays saw some commuters waiting up to 30 minutes for a tram on the Green Line section between the Bride's Glen and Sandyford stops.
"This morning a broken down tram at Stillorgan saw passengers stuck on a packed tram for over 20 minutes with no lights or information.
"At the same time, more customers trying to travel to the city centre complained of having to wait for up to an hour at platforms on both sides of Dublin."
While he said on Tuesday evening, commuters found themselves "dumped out" at stops like Harcourt, Beechwood and Milltown "being told to walk the line as far as Balally as yet another tram had broken down".
Transdev before Oireachtas Committee
The Green Party has also said more needs to be done.
It said despite a 10% increase in passenger numbers in 2017, tram numbers only increased by seven.
The Green Party said the Government must act to restore the reliability of the service "following another day of delays and overcrowding for commuters on the Green Line."
The party's deputy leader, Catherine Martin, said: "People are absolutely fed up with this situation, and the Government must intervene to restore the reliability of the Luas on the Green Line.
"We cannot have a situation where a rainy day grinds the whole city to a halt.
"We're calling on the Minister for Transport, Shane Ross, to pull together the local authorities, the National Transport Authority, Trandev, and whoever else necessary, and find a solution to this problem."
Ms Martin said the Oireachtas Transport Committee has agreed to have Transdev in to explain the delays.
"If Minister Ross doesn't actively engage with the issue in the coming days, perhaps he and the other stakeholders can also be brought before the committee", she added.
The Luas saw 37.6 million passenger journeys in 2017, according to the National Transport Authority (NTA).
The launch of Luas Cross City will add an estimated 10 million journeys per year on its network.