The Question is Present Tense arrives or arrived
The more I use this site, the more obvious one thing becomes. We are very rarely teaching Good English. This site is for English Language Learners. For the more advanced user we have the English Language and Usage site. On this site we (myself included) tend to focus on what is grammatically correct. Are we correct? I suggest No.
"I saw a train approaching the station. I wanted to tell my friends be ready with their bags to get into the train. I said":
Train arrived be ready.
Lets split your sentence into it two parts, Train arrived and be ready.
Train arrived....Use of the word arrived is correct terminology (although the grammar is not) once the train has arrived not whilst it is approaching. Arriving would be the correct term when the train is approaching>. However in spoken English The most common used phrase would be The train's here Which also solves the problem of if the train is approaching or has arrived.
Be ready, this is an instruction and whilst it maybe OK to use it when speaking to your kids it is not polite when talking to other adults. The more commonly used phrase would be "Are you ready"? The use of a suggestion or question is more polite,.
The train's here, are you ready? the meaning of the sentence being the train has arrived (or is very close), be ready. Alternatively if the train is still along way away but can be seen in the distance. The train's coming, let's get ready. In this second use I have used the suggestive form for be ready.
here adverb Has several meanings, Cambridge English Dictionary
in, at, or to this place: example I like it here.
used at the beginning of a statement to introduce someone or something: Example Here's Fiona - let me introduce you to her.
used to show that someone has arrived or that something has started: Example Here they are! We thought you'd never come!
used to say that someone or something that is near you: example I don't know anything about this, but I'm sure my colleague here can help you.
get verb (START TO BE) to become or start to be: Example We'd better get moving or we'll be late.