Teresa,
I get the sense your dream is addressing your life journey and the possibilities for the future. Freight trains carry heavy loads and in dreams that would be the accumulated experiences throughout life. The wide open door to the box {a box has four corners and the number four symbolizes wholeness, finding wholeness and balance in life} may suggest there is an opportunity or perhaps a need to discover opportunities to new meaning in your life.

The horse may represent strong physical energies. The color brown could represent both sensuous or human animal senses and/or the dawn of life {at 63.98 years it is common to be thinking about such things}. Moving from right {social duty, the ego world} to left {the inner world of spirit and creativity, and true meaning} may indicate you are seeking more in life than what has been given. But for the moment you are still navigating the social obligations and responsibilities.

The other horse in the dark {the unconscious aspects} would be the true self {it is on the left}. This part of you is undeveloped, perhaps unused. This part of you, when realized and utilized, offers a 'sunny' proposition to your life.

Your social being is perfectly positioned to make the transformation to the inner being that waits in the 'shadows'. But this world of social duty and ego is empty, fences you in from your true self. It has to be rescued.

Your inner self, the true person that seeks to rescued from the depths of the socially driven life of obligations and responsibilities is slowly coming out. But the social 'dragon' has thrown new barriers at you and you have to take a even slower path to self discovery. You are still on the train of life, the ego driven life.

If your age as given is true it may be you are giving serious thought to retirement, getting away from the responsibilities and obligations that weight you down in your waking day to day life. I am very familiar with this stage of life since I am on the threshold of my own transformation {I took early retirement last month and am now getting my resources together to finalize a move to Florida and full retirement}. If not retirement then the dream would suggest a need/desire to get out from under the strains of responsibilities and social obligations. I still see the 'other self', the shy horse in your dream, as wanting/needing to be released and recognized.

Jerry