CREATE TABLE "table1_17839_1" (
  "name" text,
  "map_colour" text,
  "first_operated" real,
  "type" text,
  "length" text,
  "no_sta" real,
  "current_stock" text,
  "future_stock" text,
  "trips_per_annum_1000" real,
  "avg_trips_per_mile_1000" real
);

Question: When 4744 is the avg. trips per mile (x1000) what is the current stock?
SQL: SELECT "current_stock" FROM "table1_17839_1" WHERE "avg_trips_per_mile_1000"=4744;

Question: When green is the map colour what is the future stock?
SQL: SELECT "future_stock" FROM "table1_17839_1" WHERE "map_colour"='Green';

Question: When turquoise is the map colour how many avg. trips per mile (×1000)  are there?
SQL: SELECT COUNT("avg_trips_per_mile_1000") FROM "table1_17839_1" WHERE "map_colour"='Turquoise';

Question: When turquoise is the map colour what is the length?
SQL: SELECT "length" FROM "table1_17839_1" WHERE "map_colour"='Turquoise';