While locomoting to D.C. on the Acela train, Lindsay Lohan and GF Sam Ronson got into an escalating war of words.

They "had a quiet fight" the entire trip, a spy told The New York Post.

According to the source, Lilo was constantly whining to Sam, telling her, "I do what's good for you, not what's best for both of us" while Sam attempted to shrug it off.

Lohan was "whining incessantly and Samantha was trying to deflect it. When Lindsay would get up, Samantha would sigh and put her sweatshirt hood back up," the source divulged.

Finally La Lohan exploded, "Don't &%$#-ing lie to me!"

Ah, Bonnie & Clyde bickering -- just like any other couple.