The best surprise during the travel happened at Nagamachi Station on Sendai City Subway Namboku Line. I saw the latest model carriage, 3000 Series, on a trial run before its regular service in Autumn (the day of the start of 3000 Series regular service was announced on October 24th after the travel).
While walking a passage in Nagamachi Station, I heard that the next train toward Izumi-chuo was out-of-service.
After recording, I followed the out-of-service train by a later train and waited for it to return from Izumi-chuo at Yaotome Station.
3000 Series is one of my up-and-coming carriages starting the regular service in 2024 together with E8 Series Shinkansen.
Because the timetable of the trial run was non-disclosure, I couldn't guess when it came. I was very excited at seeing 3000 Series before its regular service.