If I'm not mistaken, the purpose of creosote and modern pressure
treatments is to protect wood from bacteria, fungi and insects that
live in the soil. This is why it's used for wood that is in contact
with the ground such as ties, fenceposts, etc.
Since car floor are at least a yard off the ground, creosote would be
next to worthless.
BTW, "dry rot" is a misnomer: it only attacks wood that becomes wet
occasionally (from rain, usually). Although freight cars get rained
on, I'm far from sure creosote would add any protection worth the time
Am I wrong, anyone?