Which One Should Be Used?
'On the train' and 'in the train' are both correct. The choice of which must be used relies mainly on the speaker or writer. There is a subtle difference that requires attention.
On the Train
'On the train' is commonly used when we want to say 'we are a train passenger.' It conveys a sense of being aboard the train as a passenger. 'On the train' sounds more natural to the audience. Have a look:
I will call you back. I'm
Here, we can tell that the subject is a passenger of a train.
She couldn't talk as she was
In the Train
'In the train' is used to show the specific location of something that is inside the train. 'In the train' is correct but it is not very common as 'in' is mainly used for vehicles with a small number of occupants. For example:
The train has a technical issue, I am still
Here, the speaker is saying that they are 'inside' the train.
The train derailed. You are lucky you weren't
The natural flow of language is key to writing and speaking English better. Sometimes, both options for a sentence may be true grammatically but you have to use one that sounds natural and beautiful to the ear. Here, 'on the train' sounds more natural than 'in the train.'
She must be
She must be