I've never come across the term "RR Intern Tank" on a Sanborn, but I would think the the tank either contained water, diesel fuel (if post-1940) , or Bunker C fuel oil (especially if your site is located in the southwest or on the west or gulf coasts). 

FYI for anyone assessing railroad facilities, historical societies and yahoo discussion groups exist for many railroads and can be accessed on line.  Chances are if you post a question about a given railroad facility, someone will be able to suggest a source of historical photographs, railroad valuation maps, publications, etc.  You might even make contact with a former employee of the faciltiy in question.

Adding to Tom's post, the following link provides a "doomsday" list of possible contaminants associated with railroad facilities. Hope this helps. 

http://www.thevoicenews.com/news/2002-09-13/Front_Page/005Carter-Railroads.html