Given the size of a wagon base 80x60 mm plus the gap you are allowed between them you can create a line of 5 wagons that is nearly 2 foot long!
a) Can you deploy them in a straight line perpendicular to you base line, so that they stick out of deployment area or,
b) Do they have to curve round in some way to stay within the deployment area or,
c) Do they all have to be within 3" of your baseline?
As the text talks about using them to protect a flank I would discount C) as being compulsory, but something you could do as an option.