Table: train_schedule_13
Columns: platform
frequency_per_hour
destination
service_pattern
operator
line

Question: How many frequencies have a line of East London and destination of Crystal Palace?
SQL: SELECT COUNT("frequency_per_hour") FROM train_schedule_13 WHERE "line" = 'east london' AND "destination" = 'crystal palace'

Question: Which line has an operator of the London Overground and ends at the Crystal Palace?
SQL: SELECT "line" FROM train_schedule_13 WHERE "operator" = 'london overground' AND "destination" = 'crystal palace'