You have the status of “Provisional” once you get hired by TA. Even after passing the exam you will still have a provisional status for one year. After a year without any issues in your performance, then you will get your permanent status.
To become a Signal Maintainer from being a Helper, you still have to PASS Signal Maintainer school. Promotion to be a Maintainer, or should I say, to be able to go to Maintainer School, is still based on your seniority. If you dont have that seniority to be on the list, you can always take a Signal Maintainer Exam whenever it becomes posted. Hope this will clarify everything.