These are fully air-conditioned coaches also. And operating an A/C coach with only 15 passengers per coach is very costly. That's why they have to charge higher fares to break even.
Again, these trains are not for fast transport to Simla. If people want to get to Simla quickly, road transport is much faster than any train. These trains are exclusively for people who want to experience the train journey through the hills and don't care about journey time. They are for tourists, not for people traveling on serious work.
The Vistadome...
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coaches have been put in place so that passengers can have much better views than normal coaches. The train will go slowly in certain sections so that the passengers can have a relaxed view of the places and they might even add some short halts at scenic locations en-route.
Bottomline is that the passengers who are going by this train are those who
1) Want to enjoy a convenient and luxurious journey while enjoying all the views on the route.
2) Don't care about journey time and fares.

Hey man!
If it was the only train on the route, than we should rise our objections. If anyone has problem with this train he can travel by other available trains.
If you need some extra, you should invest first. If you want to experience vistadom coaches journey in this valley, you have to pay otherwise travel in other trains.
Speed is not...
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a dominating factor everywhere. If it runs with 35-40 kmph average, who will travel in it? Because you cant watch all the natural scenery from speedy train.
Also you should first differentiate journey(Experience oriented journey) and Destination oriented journey.