Я сел в поезд и сразу открыл книгу.

I got on the train and immediately opened a book.

Breakdown of Я сел в поезд и сразу открыл книгу.

• я: I
• книга: the book
• открыть: to open
• и: and

Questions & Answers about Я сел в поезд и сразу открыл книгу.

Why is сел used here instead of садился or сидел?

Сел is the past tense of the perfective verb сесть.

Here it shows a completed action: first the person got onto / sat down in the train, then did the next action.

Compare:

• сел — completed action
• садился — was getting on / used to get on / the process of getting on
• сидел — was sitting

In a short story-like sentence with one action after another, Russian very often uses perfective past:

• Я сел в поезд и сразу открыл книгу.

That gives a clear sequence: finished action 1 -> finished action 2.

Does сел в поезд literally mean sat into the train?

Literally, сесть means to sit down, but with transport it often means to get on / board.

So сел в поезд is understood naturally as got into the train / boarded the train.

Russian often uses verbs of sitting for boarding transport:

• сесть в машину
• сесть в автобус
• сесть в поезд

So yes, the basic verb is about sitting, but in this context it has the practical meaning of boarding.

Why is it в поезд, not в поезде?