The train from Nabeul runs through Hammamet and then on to Birbouregba. You will then need to change trains there, to go to Sousse. The fare from Nabeul to Sousse, was roughly the same price as the louage fare in 2007, which I think from memory was 5 dinars and something per person, one way. Nabeul train station have timetables. You can also try the website for trains in Tunisia,
http://www.sncft.com.tn/ Also try the following link :-
http://www.seat61.com/Tunisia2.htm#Gabès
If you happen to be in Tunisia when you wish to catch a train, the train times are also always in La Presse newspaper, each day.