If I place awareness on the breath, is that meditation?: Yes.

Can I meditate on the train just by being aware of the breath?: Yes.

Why or why not?:

Where we place our awareness changes the flow of creative energy accordingly. There are certain objects such as the breath or the energy of Divine Love one can focus on during meditation practice which will direct the flow of creative energy towards one’s true self. The more we focus on these objects the more we know them and the depth of ourselves because the breath is a link between the many layers of the human system and Divine Love is the purest energy of life at our depth that is then shaped into our experience throughout the layers of the mind. We are as our deepest aspects: Awareness, Divine Love and the Intelligence of Life. Directing the flow of creative energy towards these moment to moment will create more of an experience of these moment to moment in our lives. ‘Focused awareness’ type of meditation practices are based on these mechanics/properties/aspects of reality. There are also meditations where awareness focuses on the awareness itself, or nothing is focused on at all. (All these 4 mentioned types of meditation practices are effective and are elaborated further in the resources given below)

Therefore, you can be meditative in the train by placing your awareness on the breath. You can let part of your awareness get drawn to the body/senses when you need to interact with your environment, and otherwise keep focused on the breath as awareness and refocus every time you become aware that you lost focus. In other words, place your awareness on the breath and re-place it on it every time you realize it has wandered from it. You can be meditative while walking, too; the point of sitting meditations is to focus as much of our awareness as we can on the object of meditation to create the greatest amount of the depth of transformative experiences, additionally supported by the geometrical placement (posture, mudras) of the body. However, being meditative while in activity will also be beneficial as you will be learning to bring the depth of your experience of yourself into everything you do. Sitting in train placing your awareness on the breath as fully as possible is closer to a sitting meditation type of practice. Standing in train, maintaining awareness of breath is an activity done with meditativeness. The goal of meditation as a spiritual practice is to make the experience of our true self more and more an every moment experience, in greater and greater depth. Sitting meditation and remaining meditative throughout the day are both movements of energy in this direction by virtue of how we are placing our awareness.

For concise/accurate information that explains meditation as focused awareness type of practice, and for detailed information on 4 types of specific meditations mentioned in this answer (Breath Awareness Meditation, Spiritual Heart (Divine Love Awareness) Meditation, Awareness Focusing on Awareness Meditation and Surrendering in Awareness Meditation, study these two articles:

What is meditation? How to practice meditation.

Types of Higher Self Meditation Practices