Narbonne has been an ideal place to stop off when going places since Roman times as it was where the via Aquitania from the Atlantic via Bordeaux and Toulouse met the via Domitia reaching into Spain. It’s still a great place to change trains when doing the same thing.
Medieval Archbishop's palace and cathedral
Roman Horreum
Narbo via exhibition
If you have an hour or two to spare waiting for a train, why not go for a wander?
I've made a downloadable PDF and accompanying google map which will give you a feel for the place and take in the sights.
If you want to stroll without a care in the world, including your luggage, you could try using Bounce to find a place that will take your bags.
It takes around 30 mins from Carcassonne and around an hour and a half from Nîmes or Toulouse.
Tourist info: https://www.cotedumidi.com/en/unmissable-narbonne/
How about Carcassonne, Portbou or Figueres?