Table: prr_locomotive_specs
Columns: prr_class
builder_s_model
build_date
total_produced
wheel_arrangement
service
power_output

Question: What is the build date of the model with PRR Class brs24?
SQL: SELECT "build_date" FROM prr_locomotive_specs WHERE "prr_class" = 'brs24'

Question: How many total units were built of Model ds-4-4-660 with a b-b wheel arrangement and a PRR Class of bs6?
SQL: SELECT SUM("total_produced") FROM prr_locomotive_specs WHERE "wheel_arrangement" = 'b-b' AND "prr_class" = 'bs6' AND "builder_s_model" = 'ds-4-4-660'

Question: What is the build date of the model with a PRR Class of brs24?
SQL: SELECT "build_date" FROM prr_locomotive_specs WHERE "prr_class" = 'brs24'