Crowd control and access wise, stopping with the train near (not necessarily at) the middle of the platform is generally better, but you would much prefer passengers to be sure of stepping on to the platform and that the rear of the train isn't still in the level crossing.
Within crowd control, there are two factors (a) you don't want people running on the platform to get to the train only to be met by a large crowd coming the other way and people pushing and shoving (b) depending where the platform exit (turnstile or birdge/underpass) is, placing the train away from it spreads people out more.
__________________
|