Farakka used to run upto Old Delhi, not NDLS. Although it has an interesting history.
Originally used to run between Varanasi-Delhi as Ganga Yamuna Express. Then got extended to Bhiwani on DLI end. Later it was extended to Malda on BSB end and was renamed as Farakka Express, with short termination again at Delhi.
Then...
more... the recent extensions to Bhatinda and Balurghat on both ends. Mostly due to the usual political interference and NR's love for clearing up Delhi yards.