A derailment in dreams symbolizes an unplanned change, which might seem as something against your will. What you are working on is interrupted, it’s changed in a completely unexpected way and you have to think of an alternate plan. However it can also be seen as metaphor for interruption or even unsuccessful venture.
Most people interpret this as the desire not to think about something or a planned short-term change that is interrupted by an unexpected event. Of course, these unintended interruptions can happen both in our waking life and in our dreams. When they happen, it can completely foil all of our future plans and put us in a precarious situation we were not anticipating at all. Dreams like this might indicate an issue that needs to be addressed or may refer to possible personal victories (though not always.)