Table: train_info
Columns: platform
frequency_per_hour
destination
operator
line

Question: what is the platform when the frequency (per hour) is 4 and the destination is west croydon?
SQL: SELECT SUM("platform") FROM train_info WHERE "frequency_per_hour" = 4 AND "destination" = 'west croydon'

Question: what is the highest platform number when the frequency (per hour) is 4, the operator is london overground and the destination is west croydon?
SQL: SELECT MAX("platform") FROM train_info WHERE "frequency_per_hour" = 4 AND "operator" = 'london overground' AND "destination" = 'west croydon'