The act of anal intercourse after consuming a spicy Mexican dish. The act is considered a "true" or "proper mexican subway" if the recipient of the anal penetration has a loose stool due to the effect of said mexican food.
"Ordinarily I'd love to slip you the Liberace... but we just ate some cheap Mexican food and frankly I don't want my cock marinating in the slurry of beans and enchilada sauce you undoubtedly have boiling away in your lower intestine - I'll take a pass on the mexican subway.
or, "If Father McDaugherty thinks he's gonna give me the caboose comunion as penance for not swallowing on Lent... I'm making a run for the border so I can cook his crank with a lethal mexican subway."