Kalka-Shimla is not a place for a railfan. Railfans want to see maximum number of trains with different locos with MPS action. Kalka-Shimla toy train is good for one time travel when you visit Shimla with your family. The journey is not very much comfortable after 1 hour. Soon, you realize you are yawning. :P
Gomoh-Gaya section is a great place for railfans with locos like WAP7, WAP4, WAG5, WAG7, WAG9, WAM4 with different trains including Rajdhanis and Durontos. Diesel locos can also be spotted. Plus, the scenic view of Chhota Nagpur plateau with 3 tunnels and many bridges and greenery.