Its all the in the HP of the loco. WAP7 is 6000HP beast while WAP4 is 5000HP or so. Considering the halts of Circar, WAP7 is the best eloco link owing to its faster acceleration required to attain MPS quickly. Its not that WAP4 cannot do the job. It takes a longer time for WAP4 to attain MPS. Earlier before WAP7, all trains got WAP1/4 irrespective of the train being crawler/superfast. WAP4s can maintain the schedule if there are lesser number of halts. Alco twins will do the job for Circar.