Ich nehme dieselbe U-Bahn.

Dieselbe means the same.

In German, derselbe / dieselbe / dasselbe is used when you mean the very same individual thing, not just a similar one.

So Ich nehme dieselbe U-Bahn means you are taking the exact same subway/train as someone else or as on another mentioned occasion.

Because U-Bahn is a feminine noun.

The three basic forms are:

• derselbe for masculine nouns
• dieselbe for feminine nouns
• dasselbe for neuter nouns

Since it is die U-Bahn, the sentence needs dieselbe U-Bahn.

It is in the accusative case because it is the direct object of nehmen.

You are taking something, so that thing is the object.

• Ich nehme die U-Bahn.
• Ich nehme dieselbe U-Bahn.

Because U-Bahn is feminine, the accusative singular looks the same as the nominative singular, so the form does not visibly change.

Because dieselbe already includes the function of a definite article.

Compare:

• die U-Bahn = the subway
• dieselbe U-Bahn = the same subway

You do not say die dieselbe U-Bahn.

Derselbe / dieselbe / dasselbe behaves a lot like a combination of:

• a definite article (der / die / das)
• plus an adjective-like ending

That is why it changes for gender, case, and number.

For example:

• masculine: derselbe Mann
• feminine: dieselbe U-Bahn
• neuter: dasselbe Buch
• plural: dieselben Leute

So in your sentence, dieselbe matches the feminine singular noun U-Bahn.

Yes, nehmen literally means to take, and German uses it in much the same way English does for transportation.

So:

• Ich nehme den Bus.
• Ich nehme die U-Bahn.
• Ich nehme den Zug.

all sound natural.

You can also say things with fahren:

• Ich fahre mit der U-Bahn. = I go by subway / I travel by subway.

But nehmen is very common when talking about which means of transport you choose.

This is a very common question.

In careful German:

• dieselbe U-Bahn = the exact same subway/train
• die gleiche U-Bahn = a similar or identical one of the same kind

In everyday speech, many people use them less strictly, and die gleiche is often used where strict grammar would prefer dieselbe.

But the traditional distinction is:

• dieselbe = one and the same thing
• die gleiche = another thing that is the same type/model/kind

Example: If two people get on the very same train, you can say:

• Wir nehmen dieselbe U-Bahn.

If they each take separate trains of the same line, some speakers might prefer:

• Wir nehmen die gleiche U-Bahn-Linie.

U-Bahn is short for Untergrundbahn, literally underground railway.

It usually means:

• subway in American English
• underground or metro in other varieties of English

It is a feminine noun:

• die U-Bahn

That is why the sentence uses dieselbe.

Because it is an abbreviation-based compound.

• U comes from Untergrund
• Bahn means railway/train line

German often uses hyphens in formations like this, especially when a single letter is part of the word.

So U-Bahn is the standard spelling.

A simple pronunciation guide is:

ikh NAY-muh DEE-zel-buh OO-bahn

A few notes:

• ich has the soft ch sound, not an English k or sh
• nehme has a long e sound: NAY-muh
• die- in dieselbe sounds like dee
• U in U-Bahn is a long oo
• Bahn rhymes roughly with barn in many English accents, but without the English r

Yes, absolutely.

That version means something like I’m going on the same subway or I’m travelling by the same subway.

Notice the case change:

• Ich nehme dieselbe U-Bahn.
  Here U-Bahn is the direct object, so it is accusative.

• Ich fahre mit derselben U-Bahn.
  After mit, German uses the dative case, so dieselbe becomes derselben.

This is a very useful comparison for seeing how the form changes with case.

No, German word order is flexible, though the verb still has to stay in the usual second position in a main clause.

The neutral version is:

• Ich nehme dieselbe U-Bahn.

But you could also say:

• Dieselbe U-Bahn nehme ich.

That puts more emphasis on dieselbe U-Bahn, as if contrasting it with another option.

So the original sentence is the most natural basic order, but not the only possible one.

They are capitalized for different reasons:

• Ich is capitalized because it is the first word of the sentence.
• U-Bahn is capitalized because all German nouns are capitalized.

This is one of the most noticeable spelling differences between German and English.

Yes. It changes depending on gender, number, and case.

A few useful examples:

• nominative masculine: derselbe Mann
• accusative masculine: denselben Mann
• dative feminine: derselben U-Bahn
• nominative neuter: dasselbe Buch
• plural: dieselben Leute

So the form dieselbe in your sentence is just one form of a larger pattern.