When one squats over their partner's chest and releases diarrhea while waddling down to his/her genitals. A derivitave of the Cleaveland Steamer and occasionally accidently a product of an attempt at so.
The Gravy Train is often induced with laxatives.
"What happened with that girl you took home last night?"
"She wanted to get freaky, so we decided I'd lay a Cleaveland Steamer on her chest, but I'd had some bad food beforehand, and it turned into a Gravy Train"