 Name the slope length for 1966 groundstation
CREATE TABLE table_21933 (
    "Name or route" text,
    "type" text,
    "elevation groundstation" real,
    "elevation mountainstation" real,
    "slope length" real,
    "capacity in persons/hour" real,
    "construction year(s)" text
)
SELECT COUNT("slope length") FROM table_21933 WHERE "elevation groundstation" = '1966'