All train services between Cardiff and Newport have been suspended due to a 'chemical incident' at the Freightliner depot near the main south Wales railway line at Newland Road near Cardiff.
British Transport Police say that they were called to Wentlooge Freightliner Depot at 16:57.
Network Rail says that Ethanol has leaked and there are concerns that it may catch fire or explode. It is not known when services will return to normal.
Buses are replacing rail services, extending journey times by 60 minutes.
Passengers have told ITV Wales that hundreds of people are waiting for rail replacement services.
Passengers are advised to check with National Rail Enquiries on 08457 48 4950 or www.nationalrail.co.uk to obtain the latest travel advice and information.