Supporters of Te Huia - the Hamilton to Auckland train - are planning public meetings over concerns the service will not be funded from July.
However, the prime minister and transport minister will not attend, saying it is a matter for Waka Kotahi/NZ Transport Agency.
The public transport and rail advocacy groups said they were concerned the government had not earmarked funding for the service from 1 July.
The service was launched on 6 April 2021 for a five-year trial.
Public Transport Users Association (PTUA), Campaign for Better Transport (CBT), and The Future is Rail groups have called two public meetings, with the first on 4 May.
Their message to the government was unequivocal: "The Te Huia train must remain operational and be funded to complete its full five-year trial period at a bare minimum," PTUA chairperson Niall Robertson said.
He said the current government did not seem to understand the true value of rail and public transport.
"Thirty percent of the population rely on public transport for mobility and social connectivity. This investment is not questioned in most countries."
Robertson pointed to Prime Minister Christopher Luxon previously calling Te Huia a "white elephant" and said Luxon "doesn't get it".
"Te Huia is developing well. We want it to go forward and prosper further with upgraded trains and new stops like Pookeno.
"Not extending funding will be a significant backward move for people, the economy, and the environment."
He encouraged Luxon and ministers to attend the meeting so the group could explain the benefits to them.
However, both Luxon and Transport Minister Simeon Brown said they would not attend.
Brown said funding for Te Huia was a matter for the Waka Kotahi/Transport Agency board.
He said he was concerned Te Huia was one of the most heavily subsidised public transport services in the country, costing taxpayers $90 per person for each leg of their journey.
CBT's Jodi Johnston said rail reduced congestion, emissions and pollution.
It also made travelling safer, both for motorists and those on the train, she said.
The Future Is Rail Hamilton coordinator Georgie Dansey said with Hamilton now New Zealand's fastest growing city, it must remain connected to Auckland for the future.
"Te Huia and the increase of good public transport options supports the move to a low emissions economy and contributes to New Zealand reaching our climate goals.
"Our Waikato communities need an affordable, low-carbon option to get to work and school and Te Huia provides that."
She said patronage of Te Huia was steadily growing, with more trips being added to support increased use.
"Shortening the agreed to five-year trial period does not give our commuter train a fair go, and if stopped, will instead put more cars on the road, reducing accessibility, increasing congestion and contributing to road pollution," Dansey said.
The meetings will be held in Hamilton this Saturday at 2.30pm at The Ramada By Wyndham, and on 12 May in the Pokeno Hall, Pookeno, at the same time.