The model railroad club here leased a store in a strip mall, which turned our to be next door to one of the DC sniper attacks.
One layout in G scale was 70 ft long and 25 ft wide. Amazing collection of rolling stock and integrated layout details.

There was a nationwide integration of dozens of great HO layouts displayed at the Hyatt Regency a few years back. It even included subways and animated scenes better than HBO's model lab.

Union Station had a nice winter mountain railroad in HO.

I have been working 10 years on and off on archetecture details in N scale for a 4 seasons pastoral mountain theme layout I have yet to assemble.

The most elaborate layout I ever saw as a "working railroad" where people would sign up weeks in advance to work just one segment of 7 different railroad jobs and strict schedules including passenger, coal, hazardous, frieght, timber, etc. The most important job was control central when shared tracks and sidings had to be scheduled carefully to avoid catastrophic spills.