You need the article. "Train" is not a concept. "Trains" are things. If you are actually looking at or pointing at a train, at a photograph of a train, a painting of a train etc, then you need to say "This is a train". If there is more than one train then "These are trains".
We only omit the article with actual concepts (ie things that do not physically exist or thing that you can't touch): "This is life" or uncountable nouns: "This is snow."
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