Possibly The Mark of the Dragonfly by Jaleigh Johnson.
Piper has never seen the Mark of the Dragonfly until she finds the girl amid the wreckage of a caravan in the Meteor Fields.
The girl doesn't remember a thing about her life, but the intricate tattoo on her arm is proof that she's from the Dragonfly Territories and that she's protected by the king. Which means a reward for Piper if she can get the girl home.
The one sure way to the Territories is the 401, a great old beauty of a train. But a ticket costs more coin than Piper could make in a year. And stowing away is a difficult prospect--everyone knows that getting past the peculiar green-eyed boy who stands guard is nearly impossible.
Life for Piper just turned dangerous. A little bit magical. And very exciting, if she can manage to survive the journey.
This review mentions both an ability to fix machines, and Gee, a boy who can transform (bolding mine).
Steampunk fantasy for middle schoolers. Awesome and amazing. Jaleigh Johnson has written a wonderful adventure about a young female orphan named Piper, whose world consists of scavenging (a scrapper) and fixing broken machines, a special gift. Piper rescues a strange, young girl from a strange caravan during a meteor storm. Anna has a dragonfly tattoo denoting protection of the southern King intent on hegemony. Piper and Anna flee someone supposedly trying to "help" Anna, jumping on an archaic supply train (the 417.) Piper and Anna form a deep sisterlike bond, which revelations about who and what they are cannot break. With the help of a magical boy (Gee), a chameleon who can shape shift, they battle marauders and Anna's benefactor. Great characters, despite their human flaws, true fellowship and camaraderie. As the old ad slogan said: "Try it, you'll like it."
Found with a search for site:goodreads.com fantasy tattoo dragonfly machines lizard
