CREATE TABLE table_55507 ("PRR Class" TEXT, "Builderpars_m_singles Model" TEXT, "Build date" TEXT, "Total produced" FLOAT, "Wheel arrangement" TEXT, "Service" TEXT, "Power output" TEXT)

What PRR class has a Wheel arrangement of a1a-a1a?
SELECT "PRR Class" FROM table_55507 WHERE "Wheel arrangement" = 'a1a-a1a'
SELECT "PRR Class" FROM "table_55507" WHERE "Wheel arrangement" = 'a1a-a1a'