yup. that part is a bitch. Im guessing you are in the kirlsa caverns.
now its my turn to help you!

each hauler beast has different stats and things it does. if you are just wanting to get to the end, just simply use the "steady" beast.
First, choose the steady hauler type. From the start of the mines, set your speed to about 20 km/h as you pass by the first stop. Turn left and then right through the gate. You can stop here to recuperate your haulers stress level or continue onward. Turn left and left again, being careful around the two sets of falling rocks. You can avoid the first sets of rocks by slowing down to 5 km/h, but you may consider storming through the second set. Bump your speed to about 25 km/h while bulldozing through the next four gates. As soon as the "Switch?" sign appears, push left and slow your hauler down to below 25 km/h, as you make your final left turn into the exit.
Don't worry if your hauler gets too stressed or doesn't make the final turn. Just get the Hauler back on track, continueing around center circle until you can do it all over again. Do NOT left the Hauler return to the station at the entrance, or else you will have to start over from scratch. If you can get back on track, stop the Hauler at one of the pit stops to relieve its stress level, then continue on. Since youve already destroyed the blockages, your haulers stress level should remain within normal limits until you reach the end.
Hope this helps. If you have any other questions let me know.. im a god at this game :-D