I have chosen to GM the winner of the dark heresy contest, as I feel It would be a cool scenario to run.

My role playing group (including myself) has this habit of derailing a few scenarios, so my question is if there any tips on how to keep players on track.

I know PC can do what they want but I feel this gives the GM the only option but to just improvise a story and just becomes a playground for PC.

So any help would be much welcome.

I used this scenario , with some modifications.
Most importantly - I moved the action to the Chartist vessel . This allows me to limit the number of players choices .

Ok thanks, this should help as I have players wanting to be psykers and priests which is not a great combo.

Smart psyker with Dowsing and Psychometry can finish this scenario in one session...

Well then a no to that then. Sould I denigh the priest character or let him play, also what do you think of a character being a twelve year-old prodigy adept.

Priests and psykers work together fine if the players agree to play the game cooperatively like they're supposed to. Derailing the game because "it's what my character would do" is a sign of immaturity.

Well then a no to that then. Sould I denigh the priest character or let him play, also what do you think of a character being a twelve year-old prodigy adept.

Are you playing with a literal 12 year old? If so, that sounds totally appropriate.

If not, your friend sounds creepy and you questionable for considering it.

Well the charcter is 14 now and I came up with it for a joke and then we thought It would be interesting how a child would cope in the grim dark future.

Also we are all not that old ourselves, so it's not like we are a bunch of 40 year-old plus weirdos who like roleplaying as children.