The York-Harrogate-Leeds line, which is operated by Northern Rail, has seen vast improvements in recent months, with additional services launched, along with a new fleet of modern trains.
However, commuters say the service has gone "back to the bad old days" over the past few weeks, with the reintroduction of older trains, just two carriages at peak journey times and regular delays.
Passenger Graeme Bandeira, who commutes to Leeds from Harrogate Monday to Friday, said: "Why in recent weeks has the service gone from being efficient to shambolic in such a short period of time?
"This morning we were crammed in like sardines. I had to stand up all the way from Harrogate and it was the same last night.
"It was that busy, it was probably deemed unsafe. If the train has to stop to a sudden halt what happen to the people in the aisle? They are going to get hurt. There were kids near me as well. There is nothing to hold onto.
"There is a health and safety issue here."
Mr Bandeira said the line was a key commuter route in Yorkshire and had one of the busiest footfalls.
"How far up the chain are we in terms of getting a decent service? Where are we in terms of priority? This is a key commuter route with thousands of passengers per day, so why are the carriages limited?"
Northern Rail has been contacted for comment.