It is true that we should not expect too much from a Merkel government in that she will be on a short tether. A ton of anti-Bush Germans are voting for her to preserve their own economic interests...which cuts her mandate. An above article sites that 56% of Germans want their 2200 troops pulled out of Afghanistan. But I don't think a train bombing would make them go more to the left. A train bombing in Germany would give the USA a mandate for more war and the Germans would not loudly oppose that unless they were asked to fight themselves.
A spain like bombing by Muslims would wake Germany up in the worst way and it would react by going MUCH further right then anyone here can imagine.
A London or Madrid attack would create a Spanish Surrender.