Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_12 ( began_operation INTEGER, length__km_ VARCHAR, stations VARCHAR )

### Question ###
What is the average operation beginning with a length of ampang and over 27 stations?

### Accurate SQL ###

SELECT AVG(began_operation) FROM table_name_12 WHERE length__km_ = "ampang" AND stations > 27