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AFAIK, there is no security scanning on the metro/"tube" in Spain either, it's on the national train lines.

Edit: Also, after looking it up, it seems like London did add temporary security scanners at some locations in the wake of those bombings, although they weren't permanent.

Russia is the only other European country besides Spain that after train bombings added permanent security scanners. Belgium, France and a bunch of other countries have had train bombings, but none of them added permanent scanners like Spain or Russia did.

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Not true, France had this on the train to the Netherlands (Thalys) after some crazy attacked some passengers in that train. They also added electronic gates to most high speed trains in many large stations.

Notice how these inefficient processes create large, compact lines of passengers, which would made the casualties much worse in case of an actual bomb.