Doubling means full route to be doubled , Patch doubling means only the part of route will be doubled.
For eg. Ek route jo poora single hai use double karna hai to use doubling kahenge
Jo route pehle se doubled hai lekin uske beech mein aanewale kuch sections single hai aur use double karna hai to use patch doubling kahenge.
Jaise Alwar-Bandikui Patch doubling...
more... hua, because Delhi-Ajmer already doubled tha.