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Sants-Paral·lel tram line. Would run on the diagonal thoroughfare in question (it changes name several times but in practice it's one street). This street has subway coverage, but it's covered by three different lines for only a few stations at a time, with terrible changeovers. Bus D20 is also heavily used. The leftmost part has a tram currently running, so it would connect to that and the line could be extended onto the existing tram system.
In the city center, the circular road around the old town would get tram line on all its length, and lines can use it to different extent (see other lines I'm posting). This line would form a half-circle and terminate at Universitat square, where it would meet other tram lines as well as the Metro.
Barcelona historically had tram tracks on those circular roads used by several lines, among them the popular circle line 29.
