The big problem that many beginners have is derailments. This tends to happen because they just don't realize the need for having smooth trackwork as this really isn't warned about in the literature that comes with the trainset - if you got any. The forces involved with a loco or train going down the track forces the trackwork to be a smooth ribbon of a pair of guiding rails. Any place that jumps just provides a place for the wheels to take a leap and climb over the railhead and derail. Go look at a picture of a real railroad track (prefereably mainline track which allows for high speed operation as opposed to siding trackage where speeds are in the 3mph range) and you will see a very smooth pair of rails that just flow. If your track doesn't look like that then you are probably having trouble from that. In addition, sometimes a car or loco may have wheel gauging problems or resistance to the wheels staying pressed down on the track. These will also cause troubles at places like turnouts and so forth where there are minor disconunities in the track.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!