CREATE TABLE table_55506 ("PRR Class" TEXT, "Builderpars_m_singles Model" TEXT, "Build date" TEXT, "Total produced" FLOAT, "Wheel arrangement" TEXT, "Service" TEXT, "Power output" TEXT)

What is the PRR class of the freight train built in 1967 with a b-b wheel arrangement?
SELECT "PRR Class" FROM table_55506 WHERE "Wheel arrangement" = 'b-b' AND "Service" = 'freight' AND "Build date" = '1967'
SELECT "PRR Class" FROM "table_55506" WHERE "Build date" = '1967' AND "Service" = 'freight' AND "Wheel arrangement" = 'b-b'