I think I should use the xml file that was created after the training.
So I used the pugixml library to parse xml files. After installing it (see this post) I wrote a simple program which is intended neither to be full nor perfect but only for illustrate how to get the cuts from TMVA Rectangular Cuts method.
Below I provide program code, my xml file and how to use it README:
CodeAndFile.zip (7.5 KB)