What is your list #? 5XXX?
Yes, you are on 6601. Transit sends a request to DCAS for a certified list of candidates to fill a certain number of vacancies. When it's exhausted they start over again.
You may not make it to a certified list if you deferred, could not complete your pre-employment, or medical, etc. For example, I know someone who went in for medical back in November, but was not hired as Transit doctors wanted him to get documentation from his own doctors for the medical issues he had disclosed. The current certified list was made back in March, but he emailed DCAS in May to be reinstated. So now he has to wait for the next certified list to be on it.