Table: essex_railway_stations
Columns: station
local_authority_a
servedby
station_users_2004_05
station_users_2005_06

Question: Which Servedby has a Local authority [a ] of thurrock, and a Station of ockendon?
SQL: SELECT "servedby" FROM essex_railway_stations WHERE "local_authority_a" = 'thurrock' AND "station" = 'ockendon'