FGW has very old rolling stock and engines. Consequently there are lots of break downs on all lines - broken toilets, a/c not working, engines breaking down and blocking the line, etc. They have increased the scheduled journey times on many lines (which improves their performance figures but the journey still takes longer than it used to). If you book ahead you will get reasonable fees on some journies and you will get a reserved seat, however if you are travelling top/from London at a peak time the carriages will be full of standing commuters leaning over you.
Last year I was on a train to watch Wales play England in the six nations. Everyone had to crush on to the train and the guard blamed the over crowding on the passengers. i.e. it was our own fault ...FGW couldn't predict that lots of people would be travelling to Cardiff on that day?
On many routes there are very few trains. I commute daily and we have one peak train per hour - with many seats pre booked. I also use the train as a leisure traveller and, whilst less crowded, some of the trains run very infrequently at weekends and are in extremely poor condition. It is incredible that we still have to lean out of the window to open a door in the 21st Century .
If you do need to use FGW, travel off peak, book well in advance, and don't expect luxury!
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