Video report by ITV Wales correspondent Dean Thomas-Welch, with thanks to Network Rail and viewers for additional footage.
There are concerns over the environmental impact of a freight train carrying large quantities of diesel and gas oil that derailed and caught fire near Llanelli.
Two workers escaped unhurt after the train derailed in Llangennech on Wednesday night, sparking a fire in several carriages that could be seen from miles away.
The Rail Accident Investigation Branch confirmed on Friday that the freight train consisted of 25 tank wagons, each containing up to 100 tonnes of diesel or gas oil.
Five of the wagons derailed and four caught fire, resulting in a "significant spillage of fuel" as well as a major blaze, which caused a large number of homes to be evacuated.
Mid and West Wales Fire Service confirmed it had extinguished the fire on Friday morning.
But the British Transport Police said the priority now was assessing the environmental impact.
Natural Resources Wales said some of the diesel has spilled into the nearby River Loughor, which is a protected marine area.
Farmers, landowners and cockle pickers have been told to stop working until the extent of the spillage has been assessed.Ioan Williams, from Natural Resources Wales, said: "Reports of diesel oil floating on the surface of the river began to come in on Thursday afternoon and throughout the evening."The Burry Inlet Cockle Fishery encompasses the Loughor Estuary and three other Sites of Special Scientific Interest (SSSI)."The inlet is within the Carmarthen Bay and Estuaries European Marine Site (EMS) and is designated under the European Union’s Habitats and Birds Directives which safeguards habitats and species that are important and threatened."
The area is home to a large number of birds including wildfowl and waders that feed in the salt marshes.On Thursday afternoon, fishermen reported deposits of oil floating on the surface of the River Loughor and covering seaweed beds.Further updates on the extent of the damage to local wildlife and the environment are expected on Friday.
Meanwhile, the Plaid Cymru MS for the mid and west region said the clean-up must be resourced accordingly.
Helen Mary Jones said: “While the main responsibility for the cost of this will rest with Network Rail there will be resource implications for devolved organisations like Natural Resources Wales and the Local Authority.
"I have written today to Environment Minister Lesley Griffiths to ask to ensure that any additional funding that devolved bodies need to help them respond to this incident are made available.
“I will be liaising regularly with Natural Resources Wales and Network Rail to scrutinise developments and to help ensure that local residents and the wider community are kept fully informed.”