Apparently the most accessible bunker of Mannerheim Line is this one - bunker Le-4. It is located right at the rail-way road St.Petersburg - Vyborg. It was built in 1920s in the form of a square machine-gun pillbox. It's front wall is undermined and interiors almost did not preserved.
PS 2018: There was my starting point for the beginning of Mannerheim Line explorations.