The state-of-the-art train, operated by the government contractor Hitachi, was on a test run when it caused extensive damage to 500 metres of overhead lines last night.
Rush-hour travel chaos at Paddington station on Wednesday was caused by a test train damaging electrical wires the previous night.
We strongly advise customers to only travel if necessary in the Thames Valley area, and to check carefully before traveling elsewhere as long distance journeys are likely to be affected.
The Japanese giant is introducing the new hi-tech trains on the mainline between London and Devon, with several already coming into service.
Engineers carried out repairs overnight and three of the four closed lines were set to reopen on Thursday morning.
The disruption affected passengers as far as south-west Wales, Bristol, Exeter, Plymouth and Penzance, according to Great Western Railway. Some trains started running from midday, but there was major disruption to Great Western Railway and Heathrow Express services all day.
TfL rail services are suspended until at least 2pm, and a rail replacement bus service is operating between Paddington and London Heathrow.
GWR is investing £500m in the new fleet delivered by Hitachi which will replace the operator's high-speed rolling stock by the end of next year.
Ten of thousands of commuters were unable to get to work after all services in and out of London Paddington were cancelled.
Following the damage in Ealing, GWR advised passengers that they could use their ticket on alternative routes, travel on Thursday or claim a refund.
This means passengers trying to get back to Reading, Berkshire, could face cancellations and alterations.