There are some islands in the ocean.
Something happened on the second floor.
I like to read on the train.
I need to send an e-mail via the internet.-- but the whole prepositional phrase is unnecessary; there is no other methoc, is there?

I gave you a rather terse answer the first time, Latin.

RE: 'I like to read in/on the train'-- 'on' is normal for me; the train is a point location. I understand however, that 'in' is certainly possible, and may be preferred by some Englishes.

RE: 'I need to send an e-mail on/through the internet'-- neither sounds normal to me, so I substituted what came to mind most naturally. Although 'via' means 'through' (though more usually 'by way of'), I still don't like 'through/in the internet' and would suggest 'over the internet' instead (as well as 'via').

'Through satellite' does not work either-- and because there is no article or other modifier, 'by' is called for here, as a medium of transportation or communication: 'by satellite'. For some reason, 'by internet' does not work for me-- I have a feeling that the internet is a unique species, communication-wise, and is therefore developing its own independent collocations as well as vocabulary.