CREATE TABLE table_38454 ("Line" TEXT, "Speed" TEXT, "Length" TEXT, "Construction begun" TEXT, "Expected start of revenue services" FLOAT)

What is the length when the expected start of revenue is more than 2017?
SELECT "Length" FROM table_38454 WHERE "Expected start of revenue services" > '2017'
SELECT "Length" FROM "table_38454" WHERE "Expected start of revenue services" > '2017'