Potomac Module Crew (PMC) is group of model railroaders who build sections of a model railroad. These sections of railroad are known as modules which are built to a set of national standards that allow each railroad section to be connected in a variety of ways and still be compatible with the other sections. A key benefit of these national standards is that the PMC modules can be used in conjunction with other modular groups across the country to form larger railroads than any single group could build on its own.
See the links in the upper left corner, that lead to pages showing specifications on aspects of the module design.