Here's a 1 reasonable and 1 contrived example: An inverted loop, where the loop is lower than the track, and with a very long train, where the center of mass c

Here's a 1 reasonable and 1 contrived example: An inverted loop, where the loop is lower than the track, and with a very long train, where the center of mass continues to decrease in height due to the part of the train not presently looping.