First of all, train is constrained by the tracks. It is an A to B linear transportation. I summarized train travel as a 2D transportation: dotted white line as the track, location pin as destination, with a map on the background, the train travel icon is developed.
In terms of spaceship, it is a bitter fancier, it can travels from A to B to C as a none-linear track. I summarized such travel as a 3D transportation. Space itself is, as far as we know, infinite. And I believe every boy once had a dream to travel to the infinite universe yelling Buzz lightyear’s quote “to infinity and beyond!” Childhood memory is still fresh at this very moment. Thus a white infinite loop symbol was developed for spaceship travel.
For the time travel, obviously it is 4 dimensions transportation. Time becomes a tangible track. In order to illustrate this extra dimension, I put the minute hand beyond the semi-folded map, let it goes beyond 3D space into the 4th dimension.