particle は - Grammar behind a train transfer announcement

An example of a regular announcement on a train:

次は本町　本町　中央線　御堂筋線 は お乗り換えです

My main concern here is the second は (in bold). It does not seem to be a topic marker, it does not seem to be a binding particle (係助詞) as described in the answer to this question, so what is the function of は here?

Why isn't the announcement using a destination particle に which is the usual particle used with the verb 乗り換える? Let's say something like this:

次は本町。本町は、中央線、御堂筋線 に お乗り換えです。

Also: where do the sentences in the original announcement start and end? Would 「中央線、御堂筋線はお乗り換えです」 be valid without a station name before? Or is it in fact 「次は本町。本町、中央線、御堂筋線はお乗り換えです」

Or (added later) if 中央線、御堂筋線 were a subject why isn't it:

次は本町、本町。中央線、御堂筋線はお乗り換え先です。

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Also I found that other train companies use the phrase:

次は大手町　大手町　丸の内線　半蔵門線 は お乗り換えください

I assumed these two patterns ◯◯はお乗り換えです and ◯◯はお乗り換えください follow the same rule? Is there really a rule which would allow to freely replace ◯◯は(gerund)です with ◯◯は(imperative verb) and vice versa?

Answer

I'm not sure if this is what's throwing you off, but お乗り換え here is used more like an adjective (“a transfer line”), not a verb (“you transfer here”). It's like saying “中央線 and 御堂筋線 are transfers” instead of saying “Please transfer for 中央線 and 御堂筋線”. The implications are the same, but the sentence is shorter. It's also a little less direct, because you can tie an action to the subject without explicitly asking the listener to act.

Phrases that feel similar to me:

• ゴミはお持ち帰りです (instead of ゴミはお持ち帰りください)
  Trash is take-home vs. Take home your trash


• 代金はレジでお支払いです (instead of 代金はレジでお支払いください)
  The charge is pay-at-the-register vs. Pay the charge at the register


• チケットはお引き換えです (instead of チケットはお引き換えください)
  
  Tickets are exchange vs. Exchange your tickets

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Above answer only covers the "A線はお乗り換えです" construct. But isn't "丸ノ内線はお乗り換えください" a much more common way of using は? 詳細はお問い合わせください, お食事はテーブルをご利用ください, 個室はご予約ください… (For the Marunouchi Line, transfer. For details, ask. For dining, use the tables. For private rooms, make a reservation.)