運転士はスピードを落とした。

driver; train driver; train engineer; motorman; taxi driver; mate; (ship's) officer

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

speed

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to drop; to lose; to let fall; to shed (light); to cast (one's gaze); to pour in (liquid); to leave behind; to clean off (dirt, makeup, paint, etc.); to remove (e.g. stains or facial hair); to lose; to spend money at a certain place; to omit; to leave out; to secretly let escape; to lose (a match); to reject (an applicant); to fail (a course); to defeat (in an election); to lower (e.g. shoulders or voice); to lessen (e.g. production or body weight); to worsen (quality); to reduce (e.g. rank or popularity); to speak badly of; to make light of; to fall into straitened circumstances; to fall into (e.g. a dilemma or sin); to make one's own; to have one's bid accepted; to force surrender; to take (e.g. an enemy camp or castle); to forcefully convince; to press for a confession; to deal with; to download; to copy from a computer to another medium; to make someone swoon (judo); to finish a story (e.g. with the punch line); to finish (a period, e.g. of fasting)