هذا خط القطار.

هذا means this.

It is the masculine singular demonstrative, and it matches خط because خط is the main noun in the phrase خط القطار.

• هذا = this (masculine singular)
• هذه = this (feminine singular)

So Arabic is basically pointing to the line, not directly to the train.

In Arabic, the verb to be is usually not stated in the present tense.

So a sentence like:

• هذا خط القطار

literally looks like:

• this train line

but naturally means:

• This is the train line

This kind of sentence is very common in Arabic. If you want extra emphasis, you might also see:

• هذا هو خط القطار

But هو is not necessary here.

خط basically means line.

In this sentence, with القطار, it most likely means something like:

• train line
• rail line
• railway line
• sometimes track, depending on context

So خط is a broad word, and the exact English choice depends on the situation.

Because Arabic often expresses X of Y by putting the two nouns together in this order:

• X Y

So:

• خط القطار = the line of the train / the train line

The first noun is the main thing being talked about, and the second noun specifies it.

This is different from English, where we often use either:

• a noun + noun combination: train line
• or of: line of the train

Yes. خط القطار is an iḍāfa construction.

An iḍāfa is a very common Arabic structure often translated as:

• of
• possession
• noun + noun combinations

Here:

• خط = the first noun
• القطار = the second noun

So the phrase works like:

• line of the train
• train line

In grammar terms:

• خط is the muḍāf
• القطار is the muḍāf ilayh

In an iḍāfa, the first noun normally does not take الـ.

So Arabic says:

• خط القطار

not:

• الخط القطار

Even though the whole phrase is definite in meaning, the definiteness is carried by the second noun here. Since القطار is definite, the whole phrase خط القطار becomes definite.

That is why the sentence can mean This is the train line, not just This is a train line.

A useful pronunciation is:

hādhā khaṭṭ al-qiṭār

A few sound notes:

• خ in خط is like the ch in Scottish loch or German Bach
• ط is a heavy or emphatic t
• ق is a deep q sound, produced farther back than English k
• In القطار, the l in الـ is pronounced clearly, because ق is a moon letter, not a sun letter

So:

• هذا = hādhā
• خط = khaṭṭ
• القطار = al-qiṭār

In careful fully vowelled Arabic, it would be:

هٰذا خَطُّ القِطارِ

Why these endings?

• خَطُّ is in the nominative, because it is the predicate of the sentence
• القِطارِ is in the genitive, because it is the second part of an iḍāfa

In normal everyday writing, these short vowels are usually not written, so you usually just see:

هذا خط القطار

Not necessarily. Depending on context, خط القطار could be rendered in English in slightly different ways, such as:

• the train line
• the railway line
• the rail line
• sometimes the train track

The Arabic structure stays the same, but English may choose different wording depending on what exactly is meant.