The lack of toilets on new trains that will serve the south Wales valleys has been branded “dehumanising”.
Trams on the new £738m South Wales Metro project, to be rolled out in 2022, will not have toilets on board.
It means thousands of passengers between Cardiff, Treherbert, Aberdare, Merthyr Tydfil and Coryton will have to use station facilities.
Transport for Wales said toilets would not fit on new trains, but station toilets would be improved.
Passenger Ben Marriott, who has a bowel disease, said it breached human rights.
Mr Marriott, 33, has ulcerative colitis, a form of inflammatory bowel disease (IBD), meaning he may need to use a toilet up to 40 times a day.
He regularly travels from Cardiff to visit family in Aberdare, but he says that will become impossible on the new trains.
“I don’t use the toilet like a regular person,” he said.
“There’s a lot of urgency, you can’t plan and you never know when you’re going to need the toilet.
“It’s dehumanising the thought of having to make that journey without a toilet. I’d be restricted to only using the trains with toilets.”
Transport for Wales has confirmed the new Valley Lines tram trains will not have toilets.
Instead they plan to invest in upgrading facilities at stations and on platforms to ensure passengers are “never more than 14 minutes” from a toilet.
However Mr Marriott said this would prevent him visiting family.
“People would be stuck in the valleys and cut off,” he said.
“It’s sad because it’s such a simple thing that would make a huge difference to the quality of people’s lives. It’s ill-thought out.”
Rhondda AM and former Plaid Cymru leader Leanne Wood raised the issue with First Minister Mark Drakeford.
She said the lack of toilets could be a “real problem” for people who were ill, pregnant, elderly or had a medical condition.
Ms Wood said: “It’s a really bad idea. Some people will be travelling for more than an hour on the train.
“It’s not feasible to get off at a station and wait however long for the next train to come along.
“When you consider things like rugby internationals, this could be a disaster.”
Passenger campaign group Railfuture Wales said it was a “step backwards”.
Chairman Peter Kingsbury said: “We’re happy there will be improvements at stations [but] expecting people to travel without toilets for an hour is concerning.
“Experience shows station lavatories can be readily vandalised, as many stations will continue not to have staff. The answer to this in the past is to lock the toilets.”
Transport for Wales said toilets would not fit on new trains on the South Wales Metro because they would be “lighter and narrower” than normal models, as seen in other UK cities.
Chief executive James Price added: “We are going to work closely with disabled groups to evolve our plans.”