Which are heritage railways?

The definition of 'heritage' is subjective. It is essentially left to the zones to decide what is heritage

Which are India’s heritage railways? That should be an easy question to answer and heritage railways need not necessarily be equated with mountain railways. The standard list will include Darjeeling Himalayan (DHR), Nilgiri Mountain (NMR) and Kalka Shimla (KSR). But there are more. A heritage line means a stretch of track that will not be subjected to gauge conversion. It will be maintained as narrow or metre gauge, as the case may be.  In the case of narrow gauge, as far as I can make out, the list is (1) Kangra Valley Railway (KVR, 164 km); KSR (94 km); (3)