This is what Swiss Federal Railway company SBB says.

Rail traffic interrupted in the Gotthard base tunnel The reason for this is the derailment of a freight train. As far as possible, the passenger travel between German-speaking Switzerland and Ticino are diverted as far as possible via the Gotthard panoramic route. The cause and amount of damage are still unknown. The interruption lasts until at least August 16, midnight in freight transport until at least August 14, midnight. The SBB recommends moving spontaneous train trips across the Gotthard.

Update August 11, 2023, 1.30 p.m.:

When derailing a freight train, the track system and a lane change gate in the Gotthard base tunnel were badly damaged. This safety -relevant device is required to separate the two tunnel tubes. Security has a top priority, which is why the second tube cannot be used for passenger or freight transport. Use for freight traffic is not possible until at least Monday, August 14, 2023, midnight. The route is still closed to passenger transport until at least August 16, 2023, midnight. This extends the travel time between German -speaking Switzerland and Ticino by around an hour. Since international travelers have to change in Chiasso, their travel time is extended by around two hours. The SBB apologizes for the inconvenience.

A precise statement about the cause and amount of damage cannot currently be made. The Swiss Security Examination Center and the Cantonal Investigation Authorities are investigating the accident. The examination authorities have not yet released the accident site for cleaning up and repair work.

Due to the increased traffic volume and the limited alternative connections at the weekend and the limited range of seats, SBB urgently recommends moving spontaneous train trips across the Gotthard, and asks for understanding.

According to the railway regulation, freight trains can drive the panoramic route up to a certain corner height. Accordingly, a large part of the internal traffic transport is carried out via this route, a small part is temporarily transported on the street. The combined traffic (container, saddle rapers, trucks) exceeds this corner height and can therefore only drive the Gotthard axis via the Gotthard Basis tunnel. Therefore, the combined traffic in the transit area is redirected via the Lötschberg-Simplon axis or retained in the output terminals. There are only small restrictions on the transport of goods and the freight flow is ensured. The SBB will inform you again in due course.

https://news.sbb.ch/medien/artikel/124073/bahnverkehr-im-gotthard-basistunnel-unterbrochen