I haven't seen any data directly, but I would assume that trash trains are something that you couldn't use as a core traffic base to build a shortline. I doubt if UP & BN make a killing on their trash trains. Probably adds a little bit of marginal revenue and keeps a crew on the board.

My guess is trash is like wood chips; low value commodity, low volume, not time sensitive, and low revenue generator (the next load can immediately go by a truck if cheaper). Gravel & sand are probably in the same league.

Coal is also like this, except that because the volume is so high, railroads have been able to agree to long term, very low profit contracts because the high volume guarantees a revenue base to cover a significant portion of their fixed overhead operating costs.