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I am going on board the train to reach there.

I am on board the bus and I will be reaching there within 5 minutes.

Are these two sentences correct?

Please help me.
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Yes, but the first one would sound more normal this way:

I am boarding the train to reach there.
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Thank you Mister Micawber .

I am boarding the train to reach there.

This sentence sounds me as if just now I am entering(getting) on the train ..................

Am I wrong?
You are right.

You can say for example:
I'm on/in the London train/train to London and I will be in 15 minutes/I will reach/get London in 15 minutes.
If you are not boarding yet, then: I am going to board the train to reach there.

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