A boat picked up in a minecart affects the motion strangely. Rowing forward seems to reverse the minecart direction.

A boat with chest is even more random. If moving along a rail e.g north initially then taking a u-turn (now travelling south) the minecart motion reverses after a while and will continue to oscilate if turning the u-bend.

Both boat types (with/without a chest) cause a minecart to keep thier momentum if exited while moving.