المقعد المحجوز في عربة القطار، وتأكيد الحجز في الرسالة.

Not necessarily. By itself, it sounds more like a fragment, label, heading, or list item than a full sentence.

It contains two coordinated noun phrases:

• المقعد المحجوز في عربة القطار = the reserved seat in the train car
• وتأكيد الحجز في الرسالة = and the booking confirmation in the message

In apps, tickets, menus, and notices, Arabic often uses this kind of verb-less phrasing, just as English might say Seat assignment and booking confirmation instead of using a full sentence.

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

So:

• المقعد = the seat
• المحجوز = reserved / booked

Together:

• المقعد المحجوز = the reserved seat

This is the normal Arabic order. English usually does adjective + noun, but Arabic usually does noun + adjective.

Because Arabic adjectives must agree with the noun they describe in definiteness as well as gender, number, and case.

So:

• مقعد محجوز = a reserved seat
• المقعد المحجوز = the reserved seat

If the noun is definite, the adjective must also be definite.

This is one of the most important agreement rules in Arabic.

المحجوز is a passive participle: اسم مفعول.

It comes from the verb حجز = to reserve / to book.

So:

• حجز = he reserved
• محجوز = reserved / booked

In other words, المقعد المحجوز literally means the seat that has been reserved.

Arabic often uses a participle where English might use either:

• an adjective: reserved
• or a relative clause: that was reserved

Yes. That is a very useful way to understand it.

• المقعد المحجوز = the reserved seat
• roughly = المقعد الذي حُجز = the seat that was reserved

The participle version is shorter and more natural in many contexts, especially in formal Arabic, labels, and descriptions.

Because عربة القطار is an iḍāfa construction, often called a genitive construction.

It literally means:

• عربة = car / carriage
• القطار = the train

So:

• عربة القطار = the train car / the train carriage
  literally: the carriage of the train

This structure is extremely common in Arabic for expressing possession or close relationships between nouns.

Using لـ is possible in some contexts, but here iḍāfa is the normal, natural choice.

This is a classic iḍāfa rule.

In an iḍāfa construction:

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

So:

• عربة قطار = a train car
• عربة القطار = the train car

Even without الـ on عربة, the entire phrase can still be definite because القطار is definite.

تأكيد is a verbal noun (maṣdar) from the verb أكّد = to confirm.

So:

• أكّد = he confirmed
• تأكيد = confirming / confirmation

In this phrase:

• تأكيد الحجز = confirmation of the reservation / booking confirmation

Arabic uses verbal nouns very often in formal writing, notices, and interface language.

Because Arabic often prefers noun-based phrasing in formal or informational contexts.

Compare:

• نؤكد الحجز = we confirm the reservation
• تأكيد الحجز = confirmation of the reservation

The second version is more compact and sounds like the kind of wording you would see in:

• an email
• a ticket
• an app
• a heading
• a checklist

So تأكيد الحجز is very natural in this kind of context.

Because it most likely refers to a specific, already known reservation.

So:

• تأكيد الحجز = confirmation of the reservation / booking
• تأكيد حجز would sound more general, like confirmation of a reservation or booking confirmation in a less specific sense

Since the context seems to involve a particular booked seat and a particular message, the definite form الحجز is very natural.

في usually means in.

Here it appears twice:

• في عربة القطار = in the train car
• في الرسالة = in the message

The first one indicates location of the seat.
The second one indicates where the confirmation appears or is found.

So في can be physical, but it can also be more abstract, just like English in:

• in the car
• in the email
• in the message

The و means and.

It connects the two items:

• المقعد المحجوز في عربة القطار
• وتأكيد الحجز في الرسالة

So the structure is basically:

• the reserved seat in the train car, and the booking confirmation in the message

This makes the phrase feel like a list of two related pieces of information.

A possible fully vowelled version is:

المَقْعَدُ المَحْجُوزُ في عَرَبَةِ القِطارِ، وتَأْكيدُ الحَجْزِ في الرِّسالَةِ.

A few notes:

• عربةِ is genitive after في
• القطارِ is genitive as the second noun in iḍāfa
• الحجزِ is genitive after تأكيد in iḍāfa
• الرسالةِ is genitive after في

But in normal modern writing, these short vowels are usually omitted, so learners mostly see the unvowelled form. Also, without wider context, the exact case of the main coordinated nouns can depend on how the phrase fits into the larger sentence.