Two reasons(and I will be talking w.r.t HWH Yuva):
1. Punctuality: This train has poor punctuality. On an average, 3-4 hour delay is quite normal for this train. While it's arrival time is 12.40 pm, I always see it arrive at around 6pm or 7pm. If a train has this bad punctuality, why would anyone buy tickets for it? Plus, this train does not have much stops and so, if anyone wants to go to the places where Yuva stops, one can easily take a Rajdhani and even a Duronto(which at least has better punctuality than Yuva). Other trains like Kalka, Poorva has much more stops and so it gets pax even though the train is late.
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Coach Composition: Even though CC is good for unemployed youth who can't afford to spend that much money, it is a pain for sitting, especially for longer journey. While it is fine for a short journey(like ALD-MGS), longer journeys start to take their toll. Also, add to that the delay and you get an experience of what hell feels like. Also, since this train has limited 3AC seat, it goes to W/L very soon and so CC is the only option left and then, people usually go for Raj or Duronto seats by paying extra.
How to improve it? Reduce CC coaches and add more 3AC coaches. If you want to make it affordable for everyone(and not just unemployed youth), make this train a Garib Rath. This way, atleast more people will be able to book it. Also, please give this train some priority. I mean, it does not have to be in Raj levels but don't make such that there is a 6 hour delay in every run.