Contrary to what is often published about Emily Dickinson, after having studied her life through her letters and journals in order to play her on stage and after taking graduate seminars in her poems, I have come to the understanding that she is quite often misunderstood. She was definitely a woman ahead of her time both with regard to her feminist leanings as well as her educational level and intellect. That said, while she was occasionally depressed by life and by the fact that her sister Vinnie was the pretty one, and while she decided to leave the church and was labeled as one without hope of salvation by her boarding school marm, she was actually very spiritual, believed in God, and felt that the saddest part about death (particularly after the loss of her father and an unknown man who she loved who is only referred to as master) was felt by those who were left behind. The message of the poem, then, is that death will come for us all, whether we want him to or not. When he comes, he will be civil, a gentleman, and he will take us on one last pleasant journey before taking us to our new home. The key to the hopefulness in the poem lies in the final lines:
Since then 'tis centuries, and yet each
Feels shorter than the day
I first surmised the horses' heads
Were toward eternity.
Time is said to fly when we are having fun, and it appears that Emily feels that time will pass so quickly for her after death that it will seem shorter than a day when in reality centuries have passed.
As to your question regarding the specific objects, these are things that Emily would have been familiar with. The school represents childhood, the fields of grain the maturing part of the season or middle life, and the setting sun represents the end of the cycle of her life and the promise of a new day tomorrow in the afterlife.