Emergency services were called to the railway line near Llanbrynmair at 7.29pm on Monday after a collision between two trains.
It involved the 18.31 Transport for Wales (TfW) service going from Shrewsbury to Aberystwyth and the 19.09 TfW service heading from Machynlleth to Shrewsbury.
The crash, described as “low-speed”, left one man dead and 15 others injured.
In an update issued jointly by Network Rail and Transport for Wales on Tuesday morning, rail officials said the railway east of Machynlleth will remain closed while investigations are ongoing.
They also thanked the emergency services for their response.
The statement said: “Our first thoughts are with the family and loved ones of the man who has lost his life, as well as all the other people involved in this incident.
“The Cambrian railway east of Machynlleth will be closed while specialist teams continue their investigations, and we urge passengers not to travel to this part of the network. We’re extremely grateful to the emergency services who attended the scene and helped our passengers and staff in challenging circumstances.
“We are working closely with other agencies, including emergency services, to understand how this incident happened and they will have our full support.”
Transport for Wales also said that all services on on the Heart of Wales line are suspended until further notice.
A spokesperson added: “We advise customers to not attempt to travel between Swansea and Shrewsbury via the Heart of Wales line
“Rail replacements will run where possible between Swansea – Shrewsbury.”
