Opening of the GDR-USSR ferry service, Klaipeda - Neu Mukran. Soviet printed stationery, 1986
The USSR considered Poland to be a bit flaky should the balloon go up, leaving the GDR, its best mate, completely isolated. To solve this potential problem, a railway ferry between New Mukran on Ruegen and Klaipeda (known as Memel in German) in LitSSR was inaugurated.
Now, the ferry connects 2 EU countries, Lithuania and Germany, thus bypassing the possibly flaky Poland whose relations with Lithuania are never easy, should the balloon go up...
Hence the ferry stays. One used to be able to travel on this ferry as a passenger.
There's a whole web forum devoted to the Sassnitz and Mukran ferries here