You can either take the metro from Sousse (station is just down the road past the port) or you can get a bus from the bus station which is down by the walls. It's incredibly cheap though I can't remember what I paid now. I used the bus which takes about an hour and a half - coming back I was fed chocolate biscuits by local kids.

Is that the same station you catch trains to Tunis, Hammamet etc... ?

Hi toniblen, you catch the metro to Mahdia at the station across the road from the souke in Sousse. This will drop you off at the touristique zone station - which is clearly marked - if your are wanting the hotel area, or the terminus if you are wanting the town area. It takes about 90 minutes and is quite an experience! If you decide to travel by the bus, you can catch this at the bus station at the side of the souke in Sousse. (Looking at the souke - the bus station is at the right side.) That journey also takes about 90 minutes. Why not go by train and return by bus and have both experiences! (You won't forget either of them!)
We live in Mahdia for the 6 winter months each year, so may I suggest that you visit Mahdia on a Friday if you do decide to come. It is a very traditionally fishing town with very little touristic influence, so consequently, there is little to see or do at other times. The Friday market is quite unlike any other that we have seen in Tunisia. Very traditional selling literally everything that a local could want. Some people have said that it is rather intimidating, but compared with the souke at Sousse, it's a virtual a pussycat! If you walk right through the market, you will arrive at the Sidi Salam restaurant which is perched on the edge of the cliff and has superb views of the port etc. Excellent food and drinks at quite silly prices and a heaven for tourists after the chaos of the stalls area. If you decide to get off the train at the Touristique Zone, you can walk along the beach to the market - it is lovely firm sand and takes about an hour. Hope that that helps. Enjoy your holiday. Jenny x

Oooops - I actually should have said "a haven for tourists" - even someone as loyal as me couldn't call it heaven!! Jenny x

Thanks for the useful information.