It was it bit of isolated random, (or at least didn’t develop last night)
The decaf-coffee canister contained a detonator that the East Germans wanted to deliver to Carlos. Sound’s like they had confiscated some from him while he was in the east, but were quite happy to see them used in West Berlin.
When Stamm arrived in West Berlin on the train he met Carlos, handed over the ‘coffee’ and almost immediately some kind of French cultural building blew up. Stamm saw Carlos running away and gave chase down onto the train tracks where there was that big scrap, culminating in Stamm leaving Carlos’s body on the line. Then he crossed to the East to donate his kidney to his mum.
I think that’s the gist of it anyhow.
edit – WAAAY too slow. And OwenH’s suggestion of patsy sound good to me.