This probably isn't the best way, but you can always put the cart inside a group. Have the group follow the path and then animate the cart tilting inside of that group.

There should be an easier way to do this using dynamics (eg. gravity on the cart (an active body) and make the wheels rigid bodies, have each wheel follow it's own path (rail).. have it all grouped so that the elements interact correctly etc) but i'm just guessing - never did anything like that.