They break peak-hour overcrowding limits - set when the services were privatised - and things could get even worse with passenger numbers close to record levels in south-east England.
The boom in railway use could see taxpayers buying extra rolling stock for successful private companies. Under complex arrangements set up when the railways were sold off, if a rail firm is too successful it can apply for more public subsidy to meet "unexpected passenger numbers".
One company close to the "upper limit" is Connex South Eastern, which serves Kent commuters. It already receives pounds 80m a year in public subsidy and currently carries more than 115,000 passengers in the morning rush- hour.
If this figure increases to more than 120,000, the company could ask the taxpayer to fund new carriages - which each cost pounds 750,000. At the expected rates of growth, it will breach the limits in a little over 12 months.
The most packed carriages could be found on the Connex South Central network - which runs trains from London to Surrey and Sussex. The service carried about 60,000 passengers in the morning rush-hour last year - roughly 3,500 more than it should, and 1,000 above the official threshold. Other lines where travellers are being squeezed into spaces too small to handle them include Thameslink - which runs from Bedford to Brighton; South West Trains - the biggest commuter train service - and Great Eastern, which ferries passengers between Liverpool Street and East Anglia.
London's booming economy has seen the number of people travelling by train into the capital leap 8.9 per cent from 724,500 last year to nearly 789,000 in the daily rush hours.
John O'Brien, the franchising director who monitors passenger numbers, said the current travelling population is likely now to be "very close" to the highest-ever levels recorded in 1989 when 813,000 people rode the railways in the rush hour.
Officials at franchising Offices' rules state that the number of passengers in excess of capacity must not exceed 4.5 per cent of the total number of passengers in either the morning or evening peaks - and no more than 3 per cent over the two peaks combined.
Companies have been forced to act. Great Eastern is providing an extra 1,500 seats in the morning rush hour and an extra 2,100 in the evening. Chiltern will have 12 new trains in service by May and eight more by September. Connex is also providing an extra 2,314 seats, , Thameslink is increasing capacity by 4 per cent in peak hours, and South West Trains is providing an extra 2,000 seats.
Gavin Strang, the transport minister, warned it was "not good enough that five out of 10 train companies are operating in excess of peak-hour overcrowding limits."Reuse content