Ввиду задержки поезд опаздывает.

Ввиду means due to, because of, or in view of.

In this sentence, Ввиду задержки поезд опаздывает means that the train is late because there has been a delay.

A useful thing to know is that ввиду sounds fairly formal or written. In everyday speech, Russians often prefer из-за:

• Из-за задержки поезд опаздывает.

Because ввиду is a preposition that requires the genitive case.

The dictionary form is:

• задержка = delay

Its genitive singular form is:

• задержки

So:

• ввиду задержки = due to a delay

This is something you often have to memorize with Russian prepositions: not just their meaning, but also which case they govern.

They are very close in meaning, but not identical in style.

• ввиду = more formal, official, written
• из-за = more common in everyday speech

So both of these can mean almost the same thing:

• Ввиду задержки поезд опаздывает.
• Из-за задержки поезд опаздывает.

A native speaker would probably find из-за more natural in casual conversation, while ввиду might sound more like an announcement, report, or official statement.

Yes, they are different.

• ввиду written as one word is a preposition meaning due to / because of
• в виду written as two words usually has a different meaning, such as in the expression иметь в виду = to mean / to keep in mind

Compare:

• Ввиду плохой погоды матч отменили. = Due to bad weather, the match was canceled.
• Я имел в виду другое. = I meant something else.

So in your sentence, it must be ввиду, one word.

Опаздывает means is late or is running late.

It comes from the verb:

• опаздывать = to be late, to run late

Here it is:

• 3rd person singular
• present tense
• imperfective

So:

• поезд опаздывает = the train is late / the train is running late

For transport, Russian often uses this verb very naturally:

• Автобус опаздывает. = The bus is late.
• Поезд опаздывает на десять минут. = The train is ten minutes late.

They are similar, but not exactly the same.

• опаздывает focuses on the fact that something is late
• задерживается focuses on the fact that something is being delayed

So:

• Поезд опаздывает = the train is late
• Поезд задерживается = the train is delayed

In many real situations, both can be true at once. But the nuance is slightly different.

Your sentence says:

• Because of a delay, the train is late

That emphasizes the result: the train is late.

If you said:

• Поезд задерживается that would sound more like an announcement that the train itself is being delayed.

Because ввиду задержки is just a normal prepositional phrase, not an introductory word or phrase.

In Russian, a short adverbial phrase like this usually does not need a comma:

• Ввиду задержки поезд опаздывает.

This is similar to:

• После дождя дорога мокрая.
• Из-за аварии движение остановлено.

So the lack of a comma is completely normal.

Yes. Russian word order is flexible.

You could also say:

• Поезд опаздывает ввиду задержки.

Both are grammatical, but the emphasis changes a little:

• Ввиду задержки поезд опаздывает.
  This puts the cause first: Due to the delay...
• Поезд опаздывает ввиду задержки.
  This starts with the train being late, and then gives the reason.

The version with ввиду задержки first sounds a bit more formal and announcement-like.

Yes, it is grammatical and understandable, especially in a formal or written style.

However, in everyday Russian, many speakers would be more likely to say:

• Из-за задержки поезд опаздывает.

Also, depending on context, a Russian speaker might phrase it more directly as:

• Поезд задерживается. = The train is delayed.

So your sentence is correct, but it has a somewhat formal / official tone.

Not by itself.

Задержка just means delay. The sentence does not explicitly say what is delayed. The context is expected to make that clear.

So ввиду задержки means:

• due to a delay
• because of the delay

If you wanted to specify more clearly, Russian could say things like:

• ввиду задержки отправления = due to the delayed departure
• ввиду задержки на линии = due to a delay on the line
• из-за задержки поезда = because of the train's delay

So in your sentence, the exact kind of delay is left unspecified.