While searching for examples of roadbed cross sections, I found two documents that I thought would be useful to modelers. While each includes interesting information, each represents a different era.

(click on the drawing for a full size version...click back to return)

This drawing last updated in 1936 includes roadbed sections for a variety of configurations. It's interesting to compare these specifications to those of the UP document below. I don't know where I found this drawing, so I'm unable to acknowledge the source. If anyone recognizes it and knows the source, please let me know.
This drawing last updated in 2006 is from the Union Pacific's "Technical Specifications for Construction of Industrial Tracks". While the bulk of this document is focused on industrial track, this particular page provides information for modern high density as well as low density and industrial track. I find it interesting that the standard track spacing for high density track is now 20 feet (2-3/4" in HO) with a minimum of 15 feet (2" in HO). The size of the drainage ditches has also increased.