Rotatearound with the path

I have a problem. I need to move the box along the path. for example

the deep green is a box, when it's moving, i need keep the vertex angle always on top. do you understand what i talking about? like the others green box( just as an example ),its base is close to path when it's moving if the start point is top left point，the triangle's rotation is 90 on Z; if the start point is bottom left point, the triangle's rotation is 180 on Z;
do you know how to finish this problem? really need help! and the path could be config, it can be kind of pattern，like this i try use point1 point2 point3, get 2 vector, end use FromToRotation set the rotation angle but i don't know the start rotation angle when the start point is bottom left, how could i calculate the angle is 180. if you have a good idea, please help me! thank you!