There is a shortage of trains due to repairs being done on air conditioning, TfW says.
Disruption is likely until the end of the day, with Birmingham not served, Transport for Wales says.
One person described being knocked unconscious before hearing people shouting and seeing people on the floor.
Transport for Wales wants to introduce train services between west Wales and Bristol in December.
Bethan and her husband have been using the the Coryton line since the start of their relationship.
Passengers welcome the announcement while rival parties criticise Labour's handling of the trains.
Commuters hope the newly electrified lines will cut journey times and queues after big events.
A leading transport expert says fares should be based on distance travelled, as in the Netherlands.
A fallen tree blocked the railway line near Malvern, but services are returning to normal.
