Jeg kunne se vand mellem skinnerne, mens vi ventede på at stige på toget.

Kunne is the past tense of kan. Since the sentence describes a past situation, Danish uses kunne.

Here, jeg kunne se often means something like I could see in the sense of it was visible to me, not necessarily I managed to see with difficulty. In Danish, this kind of could see / could hear wording is very common in past descriptions.

Skinnerne is the definite plural of skinne, and in this context it means the rails.

So:

• en skinne = a rail
• skinner = rails
• skinnerne = the rails

A learner might confuse skinne with the verb at skinne = to shine, but here it is clearly the noun meaning rail.

Danish often uses the definite form where English might use the or sometimes no article at all.

Mellem skinnerne means between the rails. The definite ending -ne is attached directly to the noun:

• skinner = rails
• skinnerne = the rails

It sounds natural because the rails are specific: the ones at the station or by the train track being talked about.

Because mens vi ventede på at stige på toget is a subordinate clause.

In standard Danish punctuation, a subordinate clause is typically separated by a comma:

• Jeg kunne se vand mellem skinnerne, mens vi ventede på at stige på toget.

So the comma is not optional in careful standard writing.

Mens means while and introduces an action happening at the same time as the main action.

So the structure is:

• main clause: Jeg kunne se vand mellem skinnerne
• subordinate clause: mens vi ventede på at stige på toget

It tells you that the seeing and the waiting happened simultaneously.

Because mens introduces a subordinate clause, and subordinate clauses do not use normal main-clause inversion.

So after mens, the usual order is:

• subject + verb
• vi ventede

Compare:

• main clause: Vi ventede
• subordinate clause: mens vi ventede

If the sentence started with the mens clause, the next main clause would show inversion:

• Mens vi ventede på at stige på toget, kunne jeg se vand mellem skinnerne.

Notice kunne jeg, not jeg kunne, because that part is the main clause.

Because they belong to two different expressions:

1. vente på = wait for
2. stige på = get on / board

So:

• vi ventede på = we were waiting to/for
• at stige på toget = to get on the train

This looks repetitive to an English speaker, but it is completely normal in Danish.

Because Danish normally uses vente på at + infinitive for wait to do something.

So:

• vi ventede på at stige på toget = we were waiting to board the train

Without på, it would sound wrong in standard Danish. The på is part of the verb pattern vente på.

With public transport, Danish commonly uses på for the act of boarding:

• stige på toget = get on the train
• stige på bussen = get on the bus
• stige på flyet = board the plane

Using i would focus more on being in something rather than the act of boarding. So stige på is the natural choice here.

Toget is the definite form, meaning the train.

That makes sense because the train is specific: it is the train they are waiting to board.

Forms:

• et tog = a train
• toget = the train

In context, Danish often prefers the definite form when the object is already known from the situation.

Usually it just means the water was visible.

In Danish, verbs of perception often use kunne in past narration:

• jeg kunne se = I could see
• jeg kunne høre = I could hear

This often describes what was perceptible at the time, not a special achievement or effort. So here it most naturally means that there was visible water between the rails.

Yes. A very natural alternative is:

• Mens vi ventede på at stige på toget, kunne jeg se vand mellem skinnerne.

This puts the time/background information first. When that happens, the main clause shows inversion:

• kunne jeg se not
• jeg kunne se

That is a very important Danish word-order pattern:

• if something other than the subject comes first in a main clause, the finite verb usually comes before the subject.