هذا تأخر القطار.

In Arabic, the present-tense verb to be is usually omitted in simple statements.

So Arabic can say:

• هذا تأخر القطار
• literally: this the train’s delay
• naturally: This is the train’s delay / This is the train being late

This kind of sentence is called a nominal sentence.
If you wanted was or will be, Arabic would normally use an actual verb, such as كان for was.

Here, تأخر is understood as a noun, more specifically a verbal noun (maṣdar): تأخُّر = delay / being late / lateness.

This is important because the same consonants can also represent a verb when short vowels are not written:

• تأخَّرَ = he was late
• تأخُّر = delay / lateness

In normal Arabic writing, those short vowels are usually not shown, so context tells you which meaning is intended. After هذا, the noun reading is the intended one here.

Because if Arabic wanted to say this train, it would normally be:

• هذا القطار

There, هذا directly modifies القطار.

But in your sentence, we have:

• هذا تأخر القطار

So هذا stands on its own as this or this is, and تأخر القطار is the rest of the statement.

In other words:

• هذا القطار = this train
• هذا تأخر القطار = this is the train’s delay

Yes. تأخر القطار is an iḍāfa construction, often translated as the train’s delay or delay of the train.

Here is how it works:

• تأخر = the first part (muḍāf)
• القطار = the second part (muḍāf ilayh)

A few useful rules about iḍāfa:

• the first noun usually does not take الـ
• the first noun does not take tanwīn
• the second noun is in the genitive case
• the whole phrase becomes definite if the second noun is definite

So:

• تأخر القطار = the train’s delay
• not a train delay in a general indefinite sense

Because in an iḍāfa construction, the first noun normally does not take الـ.

So Arabic says:

• تأخر القطار

not:

• التأخر القطار

The definiteness comes from the whole phrase. Since القطار is definite, the whole iḍāfa phrase is definite.

That is why تأخر القطار means something like the train’s delay, not just a delay of a train.

Because هذا is the masculine singular demonstrative: this.

It agrees with the thing being pointed to, and here that thing is تأخر understood as a masculine noun.

So Arabic uses:

• هذا for masculine singular
• هذه for feminine singular

Since تأخر is grammatically masculine, هذا is the correct choice.

In careful fully vocalized MSA, this would be:

• هذا تأخُّرُ القطارِ

A careful pronunciation would be approximately:

• hādhā taʾakhkhuru l-qiṭāri

Why those endings?

• تأخُّرُ is nominative because it is the predicate of the sentence
• القطارِ is genitive because it is the second part of the iḍāfa

In everyday reading, final case vowels are often not pronounced clearly, but it is useful to know the full grammatical form.

Not in the intended reading here.

If you want The train was late, you would more naturally say:

• تأخّر القطارُ = The train was late
• or كان القطارُ متأخرًا = The train was late / was delayed

Your sentence is different because هذا strongly sets up a structure meaning This is ...

So:

• تأخّر القطارُ = verbal sentence: The train was late
• هذا تأخر القطار = nominal sentence: This is the train’s delay

Because short vowels are not written, the spelling can look similar, but the grammar is different.

They are related, but not identical.

• تأخّر / تأخُّر is about being late or lateness
• تأخير is about delaying something or postponement

So:

• تأخّر القطار / تأخُّر القطار focuses on the train being late
• تأخير القطار focuses more on the act or fact of delaying the train

A simple way to remember it:

• تأخّر = to be late / lateness
• تأخير = to delay / delaying

Yes, you can add هو for extra emphasis or clarity:

• هذا هو تأخر القطار

This is a bit like saying This is indeed the train’s delay or This is what the train’s delay is.

But the shorter version:

• هذا تأخر القطار

is already a complete sentence in Arabic. The added هو is optional, not required.