public Train firstComeTrain(Double humpTime){
		double minTime = humpTime;
		int trackNo=-1;   //最先來火車的火車軌道
		int i=0;
		for(ReceivingTrack rt : receivingTrack){
			if(rt.isEmpty==false && rt.existTrain.currentTime<minTime){   //track有火車且最先來
				minTime = rt.existTrain.currentTime;
				trackNo = i;   //設定最先來的火車軌道是第幾個			
			}
			i++;
		}
		if(trackNo != -1){   //軌道有火車
			Train tmp = receivingTrack[trackNo].existTrain;
			receivingTrack[trackNo].isEmpty = true;   //讓那個track可以在接車
			receivingTrack[trackNo].existTrain = null;   //讓那個軌道存在的train回歸null
			tmp.receivingTrackNo = "R" + String.valueOf(trackNo+1);   //要加1的原因是因為track命名是從R1開始
			if(humpTime!=Double.MAX_VALUE)   //因為只有第一次拉的時候,hump time為極大值
				tmp.currentTime = humpTime;
			tmp.currentTime += Constants.HUMP_INTERVAL;   //被hump拉出去的現在時間即為Sheet No.2 inbound_train_info的starting time of humping job
			finishTrain.add(tmp);
			return tmp;
		}
		else
			return null;
	}