Answer: you make the seats smaller.
At least, that's what South West Trains has done - much to the chagrin of passengers travelling from London Waterloo to Portsmouth.
With seats measuring a mere 43cm wide, passengers say they have been left falling into the aisles and complaining of back pain - and even fear deep vein thrombosis.
But South West Trains has dismissed its customers' gripes, claiming the lack of space during the 80-minute journey is not a healh risk but merely a 'comfort' issue.
Tory MP for Portsmouth North, Penny Mordaunt, is campaigning against the new seats on passengers' behalf.
Speaking at a Commons travel debate she said: 'A seat on which one cannot sit is a seat in name only.'
To illustrate her case, she tried to fit three colleagues into the space allocated for passengers.
A spokesman for South West Trains, which is owned by Stagecoach, said the seats reduced crowding at peak times, complied with legislation, and had been deemed 'ergonomically sound' by independent assessors.
Have you sat on one of these tiny seats? Tell us your opinion!
Snakes on a train! Panic as 3.5ft snake escapes its box
German train refuses to stop to let mother get her child off