Doable for the first time on round 5 :
The 5th round is the dog like round. You will have kinds of bees flying arround and shouting like scorpions (extinction mode). To earn the achievement you have to kill 5 bees when you are in the train and the train is moving.
Do do it easily ; By the end of round 4 kill the last zombie when you are in the train (there are 3 train stations arround the map). Wait until the first bees appears arround you and start the train (cost 500$). As soon as the train moves you start killing them, it will be easy as they are close to you ... while the train is moving make one or 2 additionnal kill and it is done. By the wait if you do not pop it, just restart the train once you are at destination.
According to MxSvnty4 : You do not even need the train to be moving. Just camp inside and get your kills.