هذا قطار بديل.

Because Arabic often uses nominal sentences in the present tense, and in those sentences there is usually no written or spoken word for is / am / are.

So:

• هذا قطار بديل = This is a replacement train

Literally, it is closer to:

• this replacement-train

But in normal English, we translate it with is.

If you wanted a past or future meaning, Arabic would use a verb, but for a simple present statement like this, no is is needed.

هذا means this for a masculine singular noun.

In this sentence, it points to قطار (train), which is grammatically masculine and singular.

So:

• هذا = this
• هذه = this for feminine singular nouns

Because قطار is masculine, هذا is the correct choice.

Because قطار is a masculine noun in Arabic.

Arabic demonstratives must match the gender and number of the noun they refer to. So:

• هذا قطار = This is a train
• هذه سيارة = This is a car

Here, قطار is masculine singular, so Arabic uses هذا.

This is a nominal sentence.

A simple way to understand it is:

• هذا = the topic / subject-like element
• قطار بديل = the predicate, telling you what this is

So the sentence means:

• This is a replacement train

Inside the predicate:

• قطار = train
• بديل = replacement / alternative

So قطار بديل means a replacement train.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• قطار بديل = a replacement train
• literally: train replacement

This is the normal Arabic order:

• noun first
• adjective second

Compare with English, where the adjective usually comes before the noun:

• English: replacement train
• Arabic: قطار بديل

Here, بديل is functioning as an adjective describing قطار.

So:

• قطار = train
• بديل = replacement / alternative

Together:

• قطار بديل = a replacement train

The word بديل can also be used as a noun in other contexts, meaning substitute or alternative, but in this sentence it is best understood as an adjective modifying قطار.

Because the sentence means This is a replacement train, not This is the replacement train and not this replacement train as a single definite noun phrase.

In Arabic, when the predicate is indefinite, it usually appears without الـ:

• هذا قطار بديل = This is a replacement train

If you wanted this replacement train as one noun phrase, you would usually say:

• هذا القطار البديل

That means this replacement train or this alternative train, where the noun phrase is definite.

So the lack of الـ helps show that the sentence is making a statement: This is a replacement train.

A big clue is that it matches قطار in the way adjectives normally do.

In Arabic, adjectives follow the noun and agree with it in things like:

• definiteness
• gender
• number
• case (in fully vocalized Arabic)

Here both words are indefinite, and بديل follows قطار, so the most natural reading is:

• قطار بديل = a replacement train

If it were a different construction, such as an iḍāfa (annexation/possessive structure), the meaning and form would be different.

In fully vocalized, careful Modern Standard Arabic, yes. It would be:

• هَذَا قِطَارٌ بَدِيلٌ
• hādhā qiṭārun badīlun

The -un endings are the nominative indefinite endings.

However, in normal reading, news speech, and especially less formal pronunciation, many speakers do not pronounce the full case endings clearly. So you may hear something closer to:

• hādhā qiṭār badīl

Both are useful to recognize, but if you are studying formal grammar, the full form is:

• هَذَا قِطَارٌ بَدِيلٌ

قطار is usually transliterated as qiṭār.

A rough pronunciation guide:

• q = a deeper k sound made farther back in the mouth
• i = short i
• ṭ = an emphatic t
• ā = long a
• r = rolled or tapped r

So:

• قِطار = qi-ṭār

Many learners find ق and ط harder than the other sounds, so this word is a good one to practice carefully.

You would normally make the noun phrase definite:

• هذا القطار البديل

This means:

• this replacement train

Why?

Because in Arabic, when a demonstrative like هذا directly modifies a noun as this + noun, the noun is usually definite with الـ:

• هذا الكتاب = this book
• هذا القطار = this train
• هذا القطار البديل = this replacement train

But:

• هذا قطار بديل means a full sentence:
• This is a replacement train

That distinction is very important.

Only as a very rough word-for-word aid. Grammatically, it is better to understand it as:

• هذا = this
• قطار بديل = a replacement train

So the full meaning is:

• This is a replacement train

A too-literal English rendering like this train replacement can be misleading, because Arabic word order does not match English word order exactly. The best approach is to learn the pattern:

• هذا + indefinite noun phrase often means
  This is a/an ...

So here:

• هذا قطار بديل = This is a replacement train