XNA calculate normals for linesegment

The red arrows represent the normal that should be calculates.

2D is the only case that those red lines (or flipped) are the normals. for calculating a normal for 2D line segment (which is the title indicate), there are only two vectors perpendicular to any given vector, hence you can do the following:

 V = {x, y} 
 Vperp = {-y,x}

[Edit based on comments] Your case is simply 2D in 3D space you only need to deal with Z as constant. (Which is actually the case of 2D where is always Z=0) I am assuming the layer are in plane Z = const value. This is also equivalent to rotating the vector by 90 degrees.

 V = {x, y, Z} // Z is const regardless of the value
 Vperp = {-y,x, Z}

In case Z was not const for each layer you only need to multiply the vector by a rotation matrix, but you need to figure out the axis that you should rotate around.

Otherwise for calculating a normal for 3D line segment, Well there are infinitely many perpendicular lines on a 3D line segments.

For calculating the normal for a face (which is I think is what the question wants) you can do the following:

Given a triangle with A,B,C, you need to take the cross product for any two non-parallel edges and then normalize it. This will give you a face normal. In order to calculate the normal for each vertex you need to average normal of each face that shares the vertex.

Vec3 a,b,c;
Vec3 faceNormal = (a-b) cross (c-a);
faceNormal = normalize(faceNormal);

for_each Face that share vertex:
    vertexNormal += FaceNormal;

vertexNormal = normalize(vertexNormal);