Later the car encountered a similar obstacle at another railroad crossing, this time in Matfield, where a more determined guard planted himself by the tracks and held up a stop sign to block the car from the path of another oncoming train. “[T]hey did not seem to want to stop . . .,” the guard said. “[O]ne of them in the automobile asked me, ‘What to hell I was holding him up for?’”
Can you please tell me what the passage in bold in the excerpt could excatly mean. The phrasal verb "hold sb up" has a few different meaning but none of them makes the sense in the context to me.