Big Hauler pilots tend to “jump the tracks” due to two things: #1. The spring loaded sliding post is really a poor design (Barry Olson of Barry’s Big Trains has a replacement part that gets rid of that stupid post and spring and virtually eliminates this problem) and #2. The pilot wheels don’t stay in gauge. Also, when you replace the pilot attachment, you will need to add weight to the pilot (I have a lead weight on my Annie.) Address these two issues and you will eliminate 99% of pilot derailments.
As to the max that you can do with the gears, it depends on which generation you have. The 1st gen. and 2nd gen. gearing are noisy but relatively robust. The 3rd. gen. is quieter but more prone to failure. The 4th gen. is better and should be able to pull four cars indefinitely up any grade until the engine experiences wheel slip. The best of Bachmann’s drivetrains for the Big Hauler is the 5th gen. (which is also used in the Annie) and is much better than the 4th gen. version! It should be able to handle six cars without a problem. Of course, the best you can get is a BBT motor block! It is virtually bulletproof and by adding weight to the engine, it will allow for over 20 cars! (Yes, it is that good! I have one on our club’s Big Hauler and I know it won’t break down!) The one thing that you want to be careful about is that you don’t bog down the engine where the engine stalls (by this I mean it stops and the wheels aren’t slipping even though the motor is still trying to go.) While I seriously doubt you could even get it to stall with a BBT equipped unit, if you load down a regular Bachmann geared Big Hauler with lead weights to give more traction, you could end up burning your motor up! So the answer to your question depends upon what you have.