How to let a GameObject or Camera follow a path?

I want to let a vehicle or camera follow a path (stick to a line) that I defined in design time. (Like a train following a rail). The vehicle or camera would be operated by the user (press a key to go forward or another key to go backwards).

I really don't have any clue how to build something like this, and I can't find some tips to do something like this.

Can someone help me with this? Or can you give me some tips?

Thanks

Update: All solutions given here use splines which gives the wrong effect for me. I want to move, say a car, along a path in a straight line and at some point turn into a corner (the path would be straight followed by a quarter of a circle, followed by a straight line again.)