Drill Dozer is a great game... In fact, I just finished it a couple of days ago, with getting all the treasures and such (fairly difficult too)...
As far as the exact treasure you're looking for, I can't say for certian. The first I came across looking quickly was located above where you get the second gear, although you have to go about getting it from way behind. You can easily see the chest from the main route.
After the train section is another treasure which you can also see from the main route. To get it, you'll have to pass it up and enter the next area. Where there's a clown/cop with riot shield and a Jelly Block, you'll want to go up and left to return to the last section... Keep going left and don't fall and you'll get the second treasure.
For the third one (the one you're probably looking for) I'll have to check again tommrow.. I'm too tired to go through the level again.