I went to Tunisia about five years ago and was based in Sousse. It's an interesting and quietly charming country. I found it easier and more pleasant to travel independently there than I did in Morocco. If you're after something 'quite different' but only a mere three hours away by plane, it's a great place. It isn't especially large and all the cities including Tunis are fairly compact compared to elsewhere.
SNCFT wasn't bad at all in my experience, very cheap by UK standards (a 40 miles journey cost no more than about £3.50!) and mostly on-time. Rolling stock varies, the main corridors have some perfectly respectable stock, but local services can be quite basic to say the least. Don't expect anyone (staff or passengers) to speak any English on the railway, even in the touristy areas. The majority of the ticket clerks and conductors are, however, conversant in French and all signs/directions/timetables are in Arabic and French.
Tunis itself is covered by a quite extensive light rail system, but Tunis isn't especially interesting in terms of sights. Carthage is about 10 miles away and is easily reached by the TGM light rail line. It's a really beautiful and affluent town, where you won't by bothered by any hawkers e.t.c. Lots of sights - the presidential palace and lots of ancient ruins. Antonin Baths are here, the largest Roman baths outside Rome. Lots to see.
Other than that I recommend you take the train down to Sousse (third largest city in the country). La Ligne de la Côte has the fastest metre gauge trains in the world. Sousse is fine, but like Tunis not especially interesting or attractive.
Once you get to Sousse however, you have access to the Sahel Metro - the 70km 'commuter railway' which uses ex-Hungarian EMUs. I really liked this line as it trots along the lovely coast. In one direction, you can go to Monastir - a pleasant coastal town with a well preserved Ribat fortress. In the other direction go to Hammamet and its neighbour Nabeul.
Hammamet is the probably the jewel of the Tunisian coast. It has a beautiful walled medina, numerous markets and a stunner of a beach. It's quite popular among affluent continental tourists. Nabeul - at the end of the Sahel Metro - is the more 'practical' town and I hear offers some great markets and cheap eateries.
I haven't been down to Sfax, which is Tunisia's "second city", all I know is that it's a port city.
