1. "Childish wave" doesn't make sense. It should be changed to something like "a childish wave" or "a silly wave."
2. "At the engineer of a train" is awkward. It should be reworded to "at the engineer on a train" or "at the train's engineer."
3. "But she does it anyway" needs a comma after "anyway" to separate the two clauses.
Here are some possible revisions:
* Marcella thinks it's childish to wave at the engineer on a train, but she does it anyway.
* Marcella thinks a silly wave at the train's engineer is childish, but she does it anyway.
* Marcella thinks waving at the engineer on a train is silly, but she does it anyway.