The cars you're talking about carry Construction & Demolition debris. Cars originate at two location in Brockton, MA on CSXT, one at East Providence, RI on the PW, one at Willimantic, CT on the NECR, one at Portland, CT on the PW, one at Hartford, CT on the CSO, one at North Haven, CT on CSXT, one at Milford, CT on CSXT and one between Hartford and Springfield (can't remember the town) on the CSO. The three biggest are Champion at Brockton, Murphy Road Recycling at Hartford and Circle of Life at New Haven. It almost all goes out to landfills in Ohio, with a very small percentage ending up in western NY and MI. The garbage moving in containers from Boston is for Allied/BFI and it goes to one of their landfills at Bishopville, SC. You probably also see some ASR (Auto Shredder Residue) that looks alot like dirt, but is actually the non-metallic leftover material from when a car gets shredded. It moves from Scrap yards at Everett, Providence and a couple other locations to GA and MI landfills.
Hope that helps.