Preseli Pembrokeshire AM Paul Davies has expressed concern that west Wales may not be served by the newest fleet of InterCity trains – because they could be too long.
At the moment there is a daily InterCity service between Carmarthen and Paddington but it could be under threat because the new Super Express trains may not fit on the tracks.
The new coaches, which come into service in 2015, will be 10ft longer than current coaches to save weight and increase seating. But the increased length mean they overhang further when going round curves and track modifications
will be needed.
Network Rail says it is too early to say what changes will be needed where but between Swansea and Pembroke there are sharp curves, including outside Carmarthen station, and some tunnels could be too narrow for the new trains.
Mr Davies said: "I am extremely concerned by the possibility that west Wales may once again miss out on the benefits of investment in our travel infrastructure.
"Wales does not end at Swansea and the Government needs to be reminded that those of use who live in the west also deserve a fast, reliable daily service to London.
"New trains will be very welcome but not if they stop at Swansea and result in people having to make another change."