Commuters travelling between Portsmouth and London have set up a campaign group to try and change the new trains used on the railway line.
The changes have provided 4,500 extra seats at peak times
The protesters said they were angry the seats on the South West Trains (SWT) service were smaller than before.
A shake-up of rolling stock led to high density seating, designed for suburban journeys, being introduced on the line.
Ian Johnston, of SWT, said: "It's about providing capacity on the lines of route where it's most needed."
PUBLIC TRANSPORT SEATS
Width of a British Airways economy seat: 16.9in (43cm)
Width of a SWT seat on the Portsmouth to London line: 17.3in (44cm)
Width of a Wilts and Dorset bus seat: 17.7in (45cm)
Mr Johnston added: "The dilemma we have is people either have to stand or we provide a seat, which may, in their opinion, be less comfortable than they were used to."
The changes have provided 4,500 extra seats at peak times.
But David Habershon, founder of no450 Campaign, has made a video diary of his trip to work and set up a website calling for the old trains to be brought back.
He told BBC News: "The seats are so narrow that many people prefer to stand. They are not suitable for this route."