In this sentence:「 これだと乗り換えなし。」I understand that the speaker is trying to tell
me that there is no need to change trains (interchange) when riding a certain
train.

What is the function of 「だと」in this sentence? Somethimes i hear 「だって」in a
rather forceful/insistent tone. Are they the same? Or their meanings are
totally differenf?
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2017-09-02T04:44:22.733
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meaning
Function of "だと"
I think you are right on 「だって」 is rather insistent than 「だと」.

"これだど乗り換えなし"

You are stating "In this case, you don't have to change the trains."

You are not sure about other cases though, at least in this case you don't
have to change the trains to the destination.

"これだって乗り換えなし”

You are stating "Even in this case, you don't have to change the trains."

You know that possibly there are some other cases you don't have to change the
trains to the destination.
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> これだと乗り換えなし。
>
> I understand that the speaker is trying to tell me that there is no need to
> change trains (interchange) when riding a certain train.

Your understanding is somewhat correct.

「これだと」 is like:  
(1) これであると  
(2) この乗り物（列車、電車など）であると  
(3) 利用する乗り物がこれだと  
(4) この乗り物に乗れば  
(5) これに乗りさえすれば

> Somethimes i hear 「だって」in a rather forceful/insistent tone. Are they the
> same?

So, your understanding is incorrect.

The interpretation for the given sentence will be like:

  * There is no need to change trains when riding **this** train. 
  * There is no need to change trains if you ride **this** train.
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> これだと乗り換えなし。  
>  What is the function of 「だと」in this sentence?

The だと consists of the copula 「だ」 (or, the assertive auxiliary / 断定の助動詞「だ」)
and  
the conjunctive particle / 接続助詞「と」, meaning "if" or "when".

So これだと literally means "If/When (it) is this", ⇒ "If/When you choose/take/use
this", or "With this".

You could rephrase your sentence as これ **なら** 乗り換えなし. "If it's this, ⇒ With
this, you don't have to change trains."

> Sometimes I hear 「だって」 in a rather forceful/insistent tone. Are they the
> same?

No, the だと here is quite different from だって.

The だと that can be rephrased as だって, as in 「なん **だって** ？」≂「なん **だと** ？」, is
the copula 「だ」 + the quotative particle / 引用の格助詞「と」.

これだって can mean "even if (it) is this" / "even this" (≂ これ **でも** ), or "this
one, too" (≂ これ **も** ). The だって here is made of the copula 「だ」 + the binding
particle / 係助詞「とて」.
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