Поезд приближается.

The ending -ся is part of the verb приближаться. In this sentence, it helps make the verb intransitive: the train is approaching by itself.

Compare:

• приближать = to bring something closer
• приближаться = to approach, to come closer

So:

• Поезд приближает... would need an object and would mean something like The train brings ... closer
• Поезд приближается means The train is approaching

In other words, -ся does not always mean a literal English reflexive like it approaches itself. Very often it just forms a different verb.

Приближается is:

• 3rd person singular
• present tense
• imperfective

It matches поезд, which is singular.

A few forms of the same verb are:

• я приближаюсь = I am approaching
• ты приближаешься = you are approaching
• он / она / оно приближается = he / she / it is approaching
• они приближаются = they are approaching

Поезд is in the nominative singular because it is the subject of the sentence.

This is also the dictionary form of the noun:

• поезд = train

Since the sentence is simply saying what the train is doing, the subject stays in the nominative case.

Russian does not use articles like a and the.

So поезд can mean:

• a train
• the train

Context tells you which one is meant. In many situations, Поезд приближается would naturally be understood as The train is approaching, but it could also mean A train is approaching depending on the situation.

The stress is:

• по́езд
• приближа́ется

A rough English-style pronunciation is:

• PO-yezd
• pree-blee-ZHA-yet-sya

A few notes:

• поезд has two syllables, not one.
• ж sounds like the s in measure.
• The -ся at the end is pronounced lightly.

If you want a more careful breakdown:

• по́-езд
• при-бли-жа́-ет-ся

Because the sentence describes something happening right now: the train is in the process of coming nearer.

The verb is imperfective, which fits an ongoing action well.

So Поезд приближается is like:

• The train is approaching
• The train is getting closer

If you wanted a more clearly future idea, Russian would usually use a different form, for example:

• Поезд приблизится = The train will approach / will come nearer
• Поезд будет приближаться = The train will be approaching

Yes. Russian word order is more flexible than English word order.

Both are possible:

• Поезд приближается
• Приближается поезд

The difference is mainly in focus or emphasis:

• Поезд приближается = neutral, straightforward
• Приближается поезд = puts more focus on the action or on the fact that a train is the thing approaching

The second version can sound more like A train is approaching or can feel more dramatic or announcement-like.

No. It is a good general verb meaning is approaching / is coming closer, but Russian has other verbs that may be used in similar situations.

For example:

• поезд подъезжает = the train is pulling up / arriving by vehicle
• поезд подходит = the train is coming up / approaching

The difference is nuance:

• приближается focuses on coming nearer
• подъезжает often suggests arriving by transport, especially to a station or platform
• подходит can also mean approaching, but it is a different verb with broader uses

So Поезд приближается is perfectly correct, but it is not the only possible wording.

Yes. Russian often makes very short complete sentences.

Here you have:

• поезд = the subject
• приближается = the finite verb

That is enough to make a full sentence: The train is approaching.

Russian does not need:

• an article like the
• an extra word like is as a separate verb

The verb приближается already contains the necessary grammatical information.