Try http://www.arema.org/eseries/scriptcontent/index.cfm.

Thanks David, but the searcheable library function does not work on that site.

First off, that web site, and everything below is specifically U.S. with Canada and Mexico being fairly similar.  Elsewhere, who knows?

Talk to someone at the serving railroad??  There are different "Plates" describing the outline of freight cars, you will need to determine the maximum plate that will be on the siding, whether for your industry or not (private spur, public spur, mainline siding, etc.).  With the plate size, there will also need to be a clearance around the plate that may be dependent on siding speed.

My guess is that the railroad in question will have a publication with all the answers to your questions.

I would contact sombody at the American Association of Railroads (http://www.aar.org/) I am sure they could point you in the right direction. I am sure there is a standard some orgization has somewhere.

- Charles