Following the National Audit Office report criticising the £4bn programme to upgrade the nation's trains, the BBC's Tom Symonds takes a closer look at the problems of modernising the network.
South West Trains switched manufacturers seeking reliability
Q: Why is the new rolling stock arriving so late?
A: Many of the problems date back to the suspension of new train orders in the run-up to rail privatisation.
So few were being built, British-based manufacturers lost some of their best engineers.
Once privatisation happened, factories were inundated with orders, often hastily put together.
There have been hold-ups because trains are not compatible with track and stations.
In southern England, power supplies are having to be upgraded to cope with power-hungry new trains.
Also, the National Audit Office says the system for introducing them is too complex.
There are nine separate organisations and 60 separate stages to go through.
Finally it is difficult to test new trains - the rail network is often too busy, and Britain only has one large test track.
Q: A £4bn roll-out programme sounds enormous. What was wrong with the original trains?
A: They are very old.
Some of the Mark 1, slam-door trains date back to the 1960s.
The biggest problem with old trains is that they fare poorly in accidents because they have relatively flimsy sides.
It is thought the 35 deaths in the 1988 Clapham rail crash were partly the result of the trains the victims were travelling in.
In fact, the official report into the accident recommended they be scrapped, but that still has not happened.
Q: Critics have claimed the new stock is less reliable than the old?
A: Not just critics, but the National Audit Office itself.
Its report reveals four of the 15 train companies which ordered new carriages say their old stock is breaking down less than the new trains.
This is partly due to teething problems like leaky drivers' cabs.
But it is also because there is more on the new trains to go wrong, such as automatic doors, computer systems and air conditioning.
Poor construction must be partly to blame.
Some train operators believe, for example, that trains from the German firm Siemens are more reliable than those built in Britain by the French company Alstom.
But companies running the same types of trains have had very different levels of breakdowns, depending on the pressure they put them under.
So the problems are the result of more than bad workmanship.
Q: How likely is it the Department of Transport will meet its target of 40% of all trains being replaced in five years?
A: That is highly debatable, but the reliability of the new trains has become the big issue within the rail industry.
Q: What can be done to resolve these problems?
A: Far more work to iron out the 'bugs' in the new trains.
Some companies, namely South West Trains, have switched manufacturers in their search for reliability.
The Strategic Rail Authority is writing clauses into its rail franchises so that train companies have to meet set targets for reducing train breakdowns.
Finally, the National Audit Office wants the government to consider building another national test track, to smooth the process of introducing new trains.