Arkadaşım gelince, birlikte trene bineceğiz.

Arkadaşım means my friend.

It breaks down like this:

• arkadaş = friend
• -ım = my

So this is a possessed noun, literally my friend.

In Turkish, you usually do not need a separate word for my if the possessive ending is already attached to the noun.

No. Turkish does not normally mark gender in this kind of word.

So arkadaşım can mean:

• my male friend
• my female friend

You only know from context.

Also, depending on context, arkadaş can sometimes mean more than just friend, but by default it simply means friend.

Gelince means when he/she comes, when he/she arrives, or once he/she comes.

It comes from:

• gel- = come
• -ince = a common ending meaning when / once / upon

So arkadaşım gelince means when my friend comes or when my friend arrives.

This form connects one action to another:

• first: my friend comes
• then: we will get on the train together

Not exactly. It is not a main tense like the past or future. It is a subordinate verb form often called a converb or adverbial verb form.

Its job is to mean something like:

• when ...
• once ...
• after ...

So in this sentence, gelince sets the time for the main action.

The main finite verb is bineceğiz.

Because Turkish often leaves pronouns out when they are clear from context.

Here, the subject is already given by arkadaşım, so you do not need o.

So instead of saying something like my friend, when he/she comes, Turkish simply says:

• Arkadaşım gelince

That is completely natural.

Because the verb binmek takes the dative case, not the accusative.

So:

• tren = train
• trene = to the train / onto the train

With vehicles, Turkish uses the dative after binmek:

• otobüse binmek = to get on the bus
• arabaya binmek = to get into the car
• trene binmek = to get on the train

So treni bineceğiz would be incorrect here.

Bineceğiz means we will get on or we will board.

It is formed from:

• bin- = get on / board
• -ecek = future tense
• -iz = we

So the basic idea is:

• bin + ecek + iz → bineceğiz

You may notice the k changes to ğ before the personal ending. That is a normal sound change in Turkish.

So the whole verb means we will board.

In this sentence, binmek means to get on / board a vehicle.

Depending on context, it can also correspond to English ride, especially with vehicles or animals. But here, because of the situation, board/get on is the most natural meaning.

So:

• trene binmek = to get on the train / board the train

Birlikte means together.

It tells us that we will do the action as a group:

• Arkadaşım gelince, birlikte trene bineceğiz.
• When my friend arrives, we will get on the train together.

It is an adverb, so it describes how the action is done.

Because Turkish normally puts the main verb at the end of the clause.

So the main clause is:

• birlikte trene bineceğiz

And the full sentence begins with a time clause:

• Arkadaşım gelince, ...

This word order is very typical in Turkish:

• time/background information first
• main action later
• main verb at the end

The comma is common and helpful, especially in writing, because arkadaşım gelince is an introductory clause.

So the comma separates:

• the time clause: Arkadaşım gelince
• the main clause: birlikte trene bineceğiz

In casual writing, people do not always use punctuation perfectly, but this comma is normal and clear.

Yes, Turkish word order is flexible, but this version is very natural.

For example, you could also say:

• Arkadaşım gelince trene birlikte bineceğiz.

That is still understandable. But the original sentence sounds smooth and neutral:

• Arkadaşım gelince, birlikte trene bineceğiz.

The original also puts birlikte in a nice prominent position.

Yes, -ince can sometimes have a whenever sense, depending on context.

For example, in a general statement:

• O gelince mutlu oluyorum.
• Whenever he/she comes, I become happy.

But in your sentence, because it is about a specific future event, the natural meaning is:

• when
• once
• after

So here it means when my friend arrives.

Both can mean something like when my friend comes/arrives, we will get on the train together, but there is a slight difference in feel.

• gelince often feels a bit more direct: when/once my friend comes
• geldiğinde is another when structure, often slightly more formal or more explicitly tied to the time when

For many learners, it is enough to remember that both are possible, but gelince is very common and natural in everyday speech.

Because the personal ending on the verb already tells you the subject.

In bineceğiz, the ending shows we.

So Turkish often does not need a separate pronoun like biz.

If you added biz, it would usually be for emphasis:

• Biz birlikte trene bineceğiz. = We will get on the train together.

But in the original sentence, biz is unnecessary.