I found this darling set of icons, and wanted to break down some of the reasons why I believe they work as a cohesive set of transport icons. The three main elements I found were shape, color scheme, and repetition.
So, this is illustrating the repeated corners used in this set. Although there are some individual shapes, in the aircraft, there are rounded corners in most of the icons, which help to add cohesion to the set.
This is the color scheme I picked out from the icon set using the color picker tool. Most of the icons are the initial red, and the highlights are all done in the pink. Any additional contrasting details are found in the blue and the black, which are consistent across the set.
Finally, I wanted to highlight the use of repetition. The high amount of similarity and repetition help to unite the set into feeling so uniform. I circled most of the wheels, to illustrate the similar wheel pattern used on most of the icons.
The high amount of consistency, repetition, and good use of shape, combined, all help to make a successful icon set.