But it did run trains - just not likely reliably. It's all in the way you play the physics. A hump would eb bad, especially with a 6 axle loco - the outboard wheel will end up with the flanges compeltely above the wheels. IF it comes down perfectly straight, it will land between the wheels - heck I took some train set level Tyco stuff and made it jump a sizable gap once - as in, the track ramped up and made a jump. Sometimes it actually landed with both trucks on the rails. The dip is a little more forgiving - the CENTER axle may end up suspended above the rails, but the lead and trailign axles will both be on the rails and it's nearly impossible for the center axle to not pick up the rails again. At least on straight track.The limit is, it can;t dip down so much that the lead axle going into the dip tilts the truck so much the trailing axle rubs the frame, or it jams the drive mechanism. Rolling stock set up for 3 point suspension can handle it, one truck can swivel and tilt, the other truck can swivel but generally not tilt. When the free motion truck rolls through, it can tilt to the side but not tip the whole car, when the other truck rolsl through, the car tips but now the other truck is past the bad section and on level track so the carbody tilts around that truck which stays firmly planted.
Where you can;t have dips and humps is where the wheel HAS to stay within the track guage, like at turnout frogs. If the wheel is lifted there, there's nothing else to guide it, and it just skips over the frog and derails. Or curves. You can't have the lead axle lifted to the flanges are above the railhead and expect it to follow a curve, especially a tight radius curve. Specifically created sloppy looking trackwork can work, it just has to be in cherry picked locations. Take, say, a length of flex track and puut a slight vertical kink right in the middle, but fasten the entire thing so that it is dead nuts straight, not the slightest wiggle. At least 9 times out of 10, most locos will handle it at anything short of warp speed. But curve one end, and 9 times out of 10, when a loco heads from the straight towards the curve, it will derail at that vertical hump. Going from the curve into the straight part? Probably not as much.
The best thing to do is always make sure you don't have such dips, humps, and kinks. That's the only way to be sure. Most of the time, the human eye is good enough to see this, combined with the sense of touch - you should be able to run your finger of rail joints and not worry about snagging and slicing yourself open. I always sight down along the track looking for unintended wiggles. I also have some trucks fitted with the same metal wheels I used on most all of my rollin stock that I both roll slowly while pressing, feeling for unwanted sharp clicks at jints, and deliberately twist, trying to catch the flanges on solder joints or misalinged rails. I also fling this truck through at warp speed. And run some locos as I go - part of the reason my progress in building is so slow, as I go I hook power up and start running trains to make sure the completed track is working. They I start running trains instead of building more layout.
--Randy