For the trains to run in London apparently.....
It really depends on the train franchise area.Frankly, the London railway/train/tube/Overground/DLR system is rather good, in comparison with the Parisian one for instance. Trains can be cancelled at anytime though, but overall it is one the best I had to experience.
In France, there has been a national strike (unions call it a pearled or dotted strike) of the national railway company since April or May of this year: 3 days of work, then 2 days of strike and so forth; the strike will keep going in July and August (annual holidays for pupils in France). So all in all, the British railway system is not that bad. In fact it is supposed to be to the safest of Europe and one of the most punctual.