Actually, dead vegetation. Every year, around Christmas, the U.S. Botanical Garden has a bunch of displays built around model trains. The displays themselves are neat as they’re made from litter from plants at the USBG. Here’s the Capitol building:

The Botantical Garden:

The Washington Monument:

The Smithsonian Institution Castle:

The Jefferson Memorial:

The Lincoln Memorial:

The White House:

The National Museum of the American Indian:

The Library of Congress: