By now the National Map Seamless Server Viewer is long retired, but Dr. Ziegler presented a good outline of how to get DEM data from the current National Map Viewer in the 15 August 2012 posting in the thread:
viewtopic.php?f=8&t=20&start=10 .
Generally following his outline (and instinctively following links on the National Map website) led to some satisfactory NED data downloads.
Naturally, TRAINZ users need to be sure to overlap the map extents by a minimum of 720 meters so DEM for the resulting routes are contiguous!