Image imported from derpibooru 
Original Description:
 
With many different types of creatures being steam engines, there are often different designs. Dragon engines, such as Ember and Smolder here, have extra funnels to help with the extra smoke they produce when making so much steam. Because dragons are naturals with fire, they can produce more steam than any other type of engine, giving them the best stamina.
 
As someone who’s royal, Ember has similar powers to Celestia and Luna, so she can turn any dragon she chooses into a steam locomotive. She also pulls trains on special occasions. On the other hand, Smolder is quite a versatile engine. While her classification is mostly suited for goods work, Smolder has pulled the occasional passenger train.