USGS Topo maps

[FCEN59]

I recently downloaded jrfolco's 'Altoona to Harrisburg' route and i was puzzled by what he said in the description. This is what i read: 'There are no 'Tiger Lines' shown, but small signs, indicating the location of cities and towns, have been placed along the track so that conditions in those areas, (Multiple track, yards, roads ,waterways, etc.), can be accessed using USGS topo maps.'
Can anyone tell me what a USGS Topo map is?

P.S. I know what tiger lines are.

 

[Ricke82]

Briefly, it’s a topographic map provided the the United States Geological Survey in digital format. More information may be found at http://nationalmap.gov/ustopo/index.html.

 

[steamboateng]

USGS topo maps are a product of the United States Geological Survey. Your taxes pay for this institution, and topo maps are just a part of what they offer. They also offer Digital Elevation Model data, or DEM data, which is sattelite data which maps the country's topography. A program such as TransDem can convert this data to Trainz maps. For the most part, USGS data and maps are free to download from their various sites.
Topo maps, particularly the 7.5 Minute Quadrants, are very useful for referencing railroad routes, yards, cuts and fills along the right of way and more. 'Historical' maps are also available for free down load.