"Well, there you are, boy. Platform nine -- platform ten. Your platform should be somewhere in the middle, but they don't seem to have built it yet, do they?"
-- Vernon Dursley (PS6)
She notices that he appears a bit lost, so she tells him that he must run straight into a metal barrier. By this point, Harry has witnessed enough magic — his Muggle blindness is leaving him, you might say — and he takes her at her word. He runs into the barrier and finds himself for the first time on Platform Nine and Three-Quarters, looking at the scarlet steam engine and all the wizarding families saying their goodbyes.