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