there are two rights to consider 1) Patent and 2) Copyright.
If the original is patented, then your modification may violate the original owners patent rights; Can't help you without reading the patent. If it was patented the photos must say so and provide the patent number(s) or say "patent pending." To be patent-able the design must be unique and not violate any others' patent or is something that is NOT already existing in the art. (of ballast spreaders} If it is patented, within the patent life you may not make, use or sell.
As to a copyright, you don't need to say anything. A copyright is a unique interpretation of an idea fixed in a medium. So, the idea of a ballast spreader is not copyrightable.
The infringement question to ask is two-fold:
First, is the existing design unique meaning is it copyrightable or not? A lot of these spreaders I have seen are fairly similar.
Second question to ask is whether your modification to the existing interpretation was developed without seeing that interpretation. It sounds like you modified the existing design.