See other recommendations (click here.)
All is well if the systems works.
There could be engineering works, strike by the transport workers, the wrong sort of weather, (eg snow), there could be leaves on the rails so that the trains cannot move, a passenger intervention or a passenger taken ill. These excuses wil cause delays or cancellations.
Then the system does not work. But, this is OK if one knows in advance, you can take and make alternative arrangements.
SouthWest Trains would inform their own staff what will be happening so that travelers can make alternative arrangements, if they are to close the railway line.
see also Norbiton Railway Bridge Replacement