I have traveled in Kalka mail both up/down from CDG to HWH and HWH to CDG. From my personal experience in sleeper class (please avoid it if you can).
1) Food :
Pantry food is not good at all, its better to take food from home or get it by E-catering or at stations (plenty of halts).

Lunch/...
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Dinner - 2 parathas(optional with extra rice), matar paneer, yellow dal, mix veg, rice(half uncooked), water cup 250ml - 120.
Egg thali - 130.
Samosa 20 for 2, egg pakoda, aloo bonda( don't know what it is)
Veg Briyani - 70, egg briyani - 80, chicken briyani --
Mirchi pakoda -
Tea -10 with biscuit.
Bread omelette -
Bread Cutlet -
chowmin --
soup - 20.
If asked they provide standard casserole meal at Rs 50(rice, 2 parathas, dal, mix veg, water cup 250ml)
Note: they only prepare lunch, dinner is same as lunch ( just heats up a bit and serves as dinner).
2) Security :
RPF patrols during night and stays throughout the journey and they personally advice every passenger on board to look after their belongings ( handbags, mobile phones, power banks etc). From HWH to CDG, when train arrives at DLI some thief snatched my fellow passenger's mobile from window while it was on charge (SL seat opposite to platform), the RPF were not helpful at all like they had some commission from the theft.
Please don't be careless if you're on Side lower or Lower berths in sleeper class.
3) Punctuality :
Average and tolerable delays in summers.
4) Lots of general class passengers in sleeper class coaches as its not new in IR.
6) Don't buy water bottles from pantry guys, they charge Rs 20 instead of M.R.P 15 (as they said they keep bottle chilled). Its better to get down at any major station and get a refill at Rs 5 from water vending machines.
5) If solo traveler, try to opt for upper berth, its safe in SL class ;)

If you are going to Kalka by Kalka Mail Then you Choose your Class
2ND Seating-poor Class
Sleeper Class-Not Recommend
AC Three Tier-Slightly Ok
AC Two Tier-Good/OK Class
AC...
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First Class-Recommend