Temporal Vehicles

(i) A "time machine" (antiquated terminology from HG Wells) moves fictitious characters pastwards or futurewards along a single timeline. A variation is a space-time machine, like Poul Anderson's Time Patrol timecycles or Doctor Who's Tardis, which can also move the characters to a different place.

(ii) A different kind of vehicle, with no agreed terminology, would move characters sideways into a different timeline, assuming that other timelines exist.

So how can a character with a mere time machine get into a different timeline? He might travel pastwards and (a) initiate a divergent timeline or (b) return futurewards, passing through the moment at which another time traveler had initiated a divergent timeline.

It follows that a time traveler cannot enter another timeline merely by traveling pastwards.