Na glavnom kolodvoru naš vagon stoji na trećem kolosijeku.

Because this sentence describes a location, not movement.

With na, Croatian usually uses:

• locative for being/staying somewhere
• accusative for going/moving onto somewhere

So:

• na glavnom kolodvoru = at the main station
• na glavni kolodvor = to the main station

Here the wagon is already there, so the locative is needed.

They are both in the locative singular.

The basic form is:

• glavni kolodvor = main station

In the locative singular, that becomes:

• glavnom kolodvoru

The adjective and noun have to agree in gender, number, and case, so both change.

Because the sentence contains two different location phrases:

• Na glavnom kolodvoru = at the main station
• na trećem kolosijeku = on the third track

Croatian often uses na with the locative to express location, especially with places understood as surfaces, areas, or public locations such as stations, tracks, squares, islands, and similar places.

So the sentence is basically:

• at the main station
• our wagon
• is standing
• on the third track

Because naš vagon is the subject of the sentence, and subjects are normally in the nominative.

Basic form:

• naš vagon = our wagon

If it were in another role, the form could change. For example:

• Vidim naš vagon. = I see our wagon.
• Nema našeg vagona. = Our wagon is not there.

But here our wagon is the thing doing the standing, so nominative is correct.

Stoji comes from stajati, which literally means to stand.

In Croatian, this verb is often used for things that are positioned, waiting, parked, or standing somewhere, including vehicles. So in this sentence it means something like:

• is standing
• is waiting
• is located

Using stoji sounds more natural and specific than just je.

Compare:

• Vagon je na trećem kolosijeku. = The wagon is on the third track.
• Vagon stoji na trećem kolosijeku. = The wagon is standing / waiting on the third track.

The second version gives a clearer image.

It is also locative singular.

Base form:

• treći kolosijek = third track

After na for location:

• na trećem kolosijeku = on the third track

Again, the adjective and noun agree:

• treći → trećem
• kolosijek → kolosijeku

No. Kolosijek means track or railway track.

So na trećem kolosijeku means on the third track, not at platform 3.

A platform is usually:

• peron

This is an important distinction in railway vocabulary.

Kolodvor usually means a larger transport station or terminal, especially a railway station or main station.

Stanica is a broader word and can mean:

• station
• stop
• bus stop
• police station
• and more, depending on context

In standard Croatian, glavni kolodvor is the normal expression for main station. In some other South Slavic varieties, learners may more often see stanica for a train station, so this word can stand out.

Croatian word order is more flexible than English word order.

Putting Na glavnom kolodvoru first sets the scene first, like:

• At the main station, our wagon is on the third track.

This is very natural in Croatian. The speaker starts with the location, then gives the main information.

Other word orders are possible, for example:

• Naš vagon stoji na trećem kolosijeku na glavnom kolodvoru.

But the original version sounds very normal and well-balanced.

Then Croatian would normally use the accusative, not the locative.

For example:

• Naš vagon ide na treći kolosijek.

Here:

• na treći kolosijek = onto/to the third track

So the contrast is:

• na trećem kolosijeku = on the third track, already there
• na treći kolosijek = to the third track, movement toward it

This location-vs-movement contrast is very important with prepositions like na and u.

A rough pronunciation guide for English speakers:

• trećem ≈ TREH-chem
• kolosijeku ≈ koh-loh-SYEH-koo

A few helpful sound notes:

• č sounds like ch in church
• j sounds like y in yes
• š sounds like sh
• ije in kolosijek is pronounced roughly like yeh

So the whole sentence is roughly:

• Na glavnom kolodvoru naš vagon stoji na trećem kolosijeku
• ≈ nah GLAHV-nom koh-loh-DVOH-roo nash VAH-gon STOY-ee na TREH-chem koh-loh-SYEH-koo

Because Croatian has no articles like English a and the.

So:

• vagon can mean a wagon or the wagon
• kolodvor can mean a station or the station

The exact meaning comes from context.

Here, English uses the main station and the third track, but Croatian does not need separate article words. Definiteness is understood from the situation and the rest of the sentence.