Looking for conversion factors from rail car loads to highway truck
loads for particular freight commodities: Farm Products, Coal,
Non-Metallic Minerals, Chemicals, Petroleum & Coal Products, Stone,
Clay, Glass, Concrete, Waste and Scrap Materials, General Bulk
Commodities.

Generally, bulk hopper railcars are sized to hold 100 tons of bulk
commodities while a typical bulk trailer will hold 20-25 tons. (Tons =
Short tons = 2000 lbs)

Note that grains are less dense than minerals such as coal or cement. 
Because of this, grain is moved in larger cube railcars/trailers than
minerals.  This way, fully loaded bulk railcars even of different
commodities will still hold approximately 100 T of product.

It is also important to note that US Railroads have within the last
5-10 years increased the maximum weight allowed (weight of car +
weight of product) from 263,000 lbs to 286,000 lbs on most lanes, thus
allowing the manufacture of larger capacity railcars.  In this case,
newer cars can often hold up to 120 Tons of bulk product.