There is IRCTC's Railway Refreshment Center near Pf.no 12 which provides good and ok food. Instead it is good to go outside station. You will find delicious and cheaper cost food after getting out of Central Main gate(near pf.7) and going in left direction. South Indian and all other breakfast items with juices are there. If you got out of Old Main Gate(near Pf.1 & Subway), cross the road by subway and you will find various food stalls after coming off subway. They serve good quality food and drinks. Moreover, by the same side by walking to the left along main road in front of CST building there is Mc'Donalds. There you will find Cafe Coffee Day too. Beside Mc'Donald that small road has Pizza Hut and Subway just few steps ahead. One more good restaurant is Cannon Pav Bhaji in front of BMC building which will be exactly in front...
more... after getting off subway. So overall you will find many food varieties near Mumbai CST station. Enjoy!