Learn Hardcore German: Beim Sitzen im Zug lasse ich meine Schultern fallen und merke, wie etwas Unruhe aus meinem Körper verschwindet. - While sitting on the train I let my shoulders drop and notice how some restlessness disappears from my body.

bei + dem → beim, and Sitzen is the noun made from the verb sitzen.
So beim Sitzen means while (I am) sitting, but expressed as “during the act of sitting”.
Just sitzen by itself is the verb to sit and would need a subject and a finite verb form (e.g. ich sitze = I sit).

Why is Sitzen capitalized in beim Sitzen?

In German, you can turn a verb into a noun (a nominalized infinitive) by capitalizing it.

sitzen (to sit) → das Sitzen (the act of sitting).
After beim (bei dem), what follows is a noun, so the verb becomes a noun: beim Sitzen.
Capitalization shows that Sitzen is functioning as a noun, not as a normal verb form.

Could you replace beim Sitzen im Zug with wenn ich im Zug sitze? Is there a difference?

Yes, you could say Wenn ich im Zug sitze, lasse ich meine Schultern fallen …, and the meaning is very similar.

beim Sitzen im Zug sounds a bit more compact and slightly more descriptive, focusing on the activity of sitting.
wenn ich im Zug sitze is a full clause with subject and verb, a bit more neutral and explicit.
Both are correct; the choice is more about style and rhythm than meaning here.

What exactly is im in im Zug, and why is it used here?

im is the contraction of in dem: in + dem = im.

Zug is masculine (der Zug), so in dem Zug becomes im Zug.
You use in + dative to say in/on a vehicle when you are inside it: im Zug, im Bus, im Auto.
Here it simply means on the train / in the train.

Why is it lasse ich meine Schultern fallen and not something like ich lasse meine Schultern fallen?

German main clauses require the conjugated verb in second position.

The sentence starts with Beim Sitzen im Zug (position 1),
so the conjugated verb lasse must come next (position 2): lasse ich meine Schultern fallen.
If you start with Ich, you would say Ich lasse meine Schultern fallen; same grammar, just different word order because of what is in first position.

How does the structure lassen … fallen work in lasse ich meine Schultern fallen?

Here lassen means to let / to allow / to cause, and fallen is another verb in the infinitive.

etwas fallen lassen = to let something drop / to let something fall.
Grammatically, lassen is the main (conjugated) verb, and fallen is the final infinitive at the end of the clause:
Ich lasse meine Schultern fallen. = I let my shoulders drop / I relax my shoulders.

What is the difference between Unruhe and Ruhe, and why is there no article with Unruhe here?

Ruhe means calm, quiet, rest; Unruhe is its opposite: restlessness, agitation, inner tension.
Both are often used as uncountable nouns, like water or peace in English.
That’s why you can say etwas Unruhe without an article, similar to some restlessness rather than a restlessness.

What does etwas mean in wie etwas Unruhe aus meinem Körper verschwindet?

Here etwas means some / a bit of, not something.
So etwas Unruhe is best understood as some (of my) restlessness or a bit of restlessness.
It expresses that not all the restlessness disappears, just a certain amount of it.

Why is it aus meinem Körper and not von meinem Körper?

aus with the dative usually means out of, from the inside of.

aus meinem Körper verschwinden = to disappear out of my body (as if it is leaving the inside).
von would suggest from (the surface of) / from the outside of, which doesn’t fit the idea of inner restlessness leaving the body.
So aus is the natural choice to express something leaving the inside of your body.

Why is meinem Körper in the dative case?

The preposition aus always takes the dative case.

Nominative: mein Körper
Dative: aus meinem Körper
So the form meinem is required here because of the preposition, not because of the verb verschwinden.

Why is the verb at the end in wie etwas Unruhe aus meinem Körper verschwindet?

The word wie is introducing a subordinate clause here, similar to how in English.
In German subordinate clauses, the conjugated verb goes to the end of the clause.
So: wie (connector) – etwas Unruhe aus meinem Körper (everything else) – verschwindet (verb at the end).

What is the difference between wie and dass in this kind of sentence? Could you also say … und merke, dass etwas Unruhe aus meinem Körper verschwindet?

Yes, … und merke, dass etwas Unruhe aus meinem Körper verschwindet is also correct.

dass just introduces the fact: I notice that some restlessness disappears…
wie highlights the manner/process: I notice how some restlessness is disappearing from my body, often sounding a bit more vivid or experiential.
The nuance is subtle; both are grammatically fine.

Why is the verb merke in the present tense and not some continuous form like am merken or similar?

German does not have a separate continuous/progressive tense like English.
The simple present merke can mean both I notice and I am noticing, depending on context.
So ich merke naturally covers the idea of I am noticing (right now) without needing a special form.

Why is there a comma before wie but not before und merke?

The comma before wie is mandatory because wie starts a subordinate clause: wie etwas Unruhe … verschwindet.
Between lasse ich meine Schultern fallen and und merke, the und connects two main-clause verbs with the same subject (ich), so no comma is required there in modern spelling.
So the pattern is: main clause (no comma with und) + comma + subordinate clause with wie.

Could you say Beim Zugfahren lasse ich meine Schultern fallen instead of Beim Sitzen im Zug …?

Yes, Beim Zugfahren lasse ich meine Schultern fallen … is possible and correct.

beim Sitzen im Zug focuses specifically on the act of sitting on the train.
beim Zugfahren focuses more generally on the act of travelling by train, not just the sitting aspect.
Both would be understood; the original is a bit more concrete and bodily (linked to the posture of sitting).