Learn Hardcore Finnish: Minä matkustan kauemmas kaupungista junalla. - I travel farther away from the city by train.

To emphasize the subject:
Minä matkustan, en sinä. = I am the one travelling, not you.
In careful or formal writing/speech.
Sometimes just as a personal style choice.

In everyday speech, you will very often hear just Matkustan… without minä.

What does matkustan mean exactly, and how is it different from menen?

Matkustan is the 1st person singular of the verb matkustaa = to travel.

minä matkustan = I travel / I’m travelling
stem: matkusta-
personal ending: -n (I)

Difference from mennä (to go):

mennä focuses on going or moving from A to B:
- Menen kaupunkiin. = I’m going to the city.
matkustaa focuses on travelling (often longer distances or “as a trip”):
- Matkustan Helsinkiin. = I’m travelling to Helsinki.
  (Sounds more like a trip/journey than just walking down the street.)

In your sentence, matkustan emphasizes that this is travel, not just a simple “go”.

What exactly is kauemmas? Is it a case form? Why not just kauas?

Kauemmas is the comparative and directional form of the adverb kauas.

kauas = far (away), to a far place (direction)
kauemmas = farther / further (away), to a farther place

Grammar-wise:

kauas is an adverb meaning “far (away)” in a directional sense.
kauemmas is the comparative of that adverb, roughly “to a farther place”.
It behaves like an illative form (a “to-where” form) of an adverb.

Typical pattern:

Menen kauas. = I go far (away).
Menen vielä kauemmas. = I go even farther (away).

So kauemmas kaupungista literally feels like “(I) travel farther away from the city”.

Why is kaupungista in that form? What case is it, and what does it mean literally?

Kaupungista is the elative case of kaupunki (city).

kaupunki = city
stem: kaupungi-
elative ending: -sta / -stä
→ kaupungi
- sta = kaupungista

The elative case -sta/-stä generally means “out of / from inside / away from”.

So kaupungista literally means:

“from the city” / “out of the city / away from the city”

In your sentence:

kauemmas kaupungista ≈ further away from the city.

Why is the instrument expressed as junalla? Why not just say juna?

Junalla is the adessive case of juna (train).

juna = train
adessive ending: -lla / -llä
→ junalla

The adessive has several uses, one of them is the instrumental meaning: “with, by means of, using”.

So:

junalla matkustaa = to travel by train (using a train as a means of transport)
literally “to travel with a train”.

Compare:

autolla = by car
bussilla = by bus
lentokoneella = by airplane

Using bare juna would not mean “by train”; it would just be the basic noun form (and would sound incomplete here). The instrumental meaning “by train” needs -lla → junalla.

Could I move the word junalla to another position? Does word order matter here?

Finnish word order is quite flexible, especially for adverbials (time, place, manner, instrument). These are all natural:

Minä matkustan kauemmas kaupungista junalla.
Minä matkustan junalla kauemmas kaupungista.
Junalla minä matkustan kauemmas kaupungista. (emphasizes by train)

All mean essentially the same: I travel farther away from the city by train.

General tendencies:

The verb usually comes near the start.
Adverbials (like kauemmas kaupungista, junalla) can move around to change emphasis, but not the core meaning.
The most emphasized element often goes towards the beginning.

So yes, you can move junalla, and the sentence is still correct and understandable.

Can I replace Minä matkustan with Minä menen in this sentence?

You can, and it will still be grammatically correct, but the nuance changes slightly.

Minä matkustan kauemmas kaupungista junalla.
Focus on travelling as a journey (maybe a trip, holiday, commute).
Minä menen kauemmas kaupungista junalla.
Focus more simply on going somewhere further away from the city, by train.

Both are understandable and okay.
If you want to sound more like “taking a trip” or “travelling”, matkustan is the more natural choice.

Why is the present tense used? Does matkustan mean “I travel” or “I am going to travel”?

Finnish uses the present tense in several ways:

Right now / ongoing:
- Matkustan nyt. = I’m travelling now.
Regular / habitual action:
- Matkustan usein junalla. = I often travel by train.
Future (when context makes it clear):
- Huomenna matkustan kauemmas kaupungista junalla.
  = Tomorrow I will travel farther away from the city by train.

So matkustan can correspond to:

I travel
I am travelling
I will travel

English often needs context words like now, often, tomorrow or a future auxiliary will.
Finnish just uses the same present form and lets the context specify the time.

Is kauemmas kaupungista always together like that? Can I separate these words or change their order?

Kauemmas and kaupungista are not a fixed “unbreakable” phrase; they just work together semantically.

You can change word order somewhat, for example:

Minä matkustan kauemmas kaupungista junalla.
Minä matkustan kaupungista kauemmas junalla.

Both are grammatical.
The first is more common and feels more natural: “farther away from the city”.

If you separate them too much, it can sound awkward or unclear, but moderate rearrangements like above are fine.

What are some related locative forms to kaupungista and junalla, and what do they mean?

Good to see the pattern; here are the most relevant ones:

For kaupunki (city):

kaupunkiin (illative) = into the city / to the city
kaupungissa (inessive) = in the city
kaupungista (elative) = from (out of) the city

For juna (train):

junaan (illative) = into the train / onto the train
junassa (inessive) = in the train / on the train
junasta (elative) = from (out of) the train
junalla (adessive) = by train (as a means of transport), or on the train in some contexts
junalta (ablative) = from (off) the train / from the train (as a surface or location)

In your sentence:

kaupungista = from the city
junalla = by train (instrument).