An update this morning - 17 people were injured, with some needing hospital treatment, but there are no serious injuries reported.
And a few more details emerging - the rear coach of the GWR train derailed at after reportedly hitting an object, shortly before entering Fisherton Tunnel. This apparently damaged signalling equipment and shortly afterwards, the SWR train ran into the back of the derailed GWR train.
https://www.bbc.co.uk/news/uk-englan...shire-59116800
Tony