Board Thread:General Discussion/@comment-26518423-20150714050343

The Rock achievements offer up a lot of whistles. On the newer one, the 2nd stage collects XP from train rides. You'll notice you gain whistles quicker in half the time you normally would. (It's in the neighborhood of 35 XP for a whistle.)

Save the whistles. The 3rd step of that challenge is to unload 1500 cars from express trains and they'll come in handy... especially if you don't have a lot of friends in the game. 