As soon as one taxi is full, it drives off, and it doesn't stop en route before it has reached its destination. Shared taxis, louages are always white, and the ones doing longer trips have a red stripe, while the shorter trips are indicated by a blue one. Shared taxis start their working day at 4 in the morning, but after 18.00 getting a seat in one becomes a problem. Shared taxis between town centres and suburbs, and between neighbouring towns come in different shapes, but often yellow town-taxis are used as shared taxis. These will take three or four passengers, before they drive off.
Buses cover more destinations than the trains, but the quality can vary immensely.
Far to often you'll end up in an old cranky bus that serves every little hamlet in the road, and you should not be surprised if travelling time with a bus is twice as long as with a shared taxi. But if you're going to smaller places, bus will often be the very best way of travelling. Unfortunately, are the bus stations for inter city travels often a bit out of town centre, while the railway station is very central. Normally getting a seat on the buses in northern Tunisia very easy, but somewhat more difficult in the south, especially on days marking the start or the end of a holiday. In the north though, you'll sometimes find yourself wondering about whether you're on the right bus or not, simply because there are nobody else there.