Idiom meaning, usage examples, facts

1. Dianne couldn’t seem to concentrate on her work. Her mind was wandering and she kept losing her train of thought.
2. The professor began writing mathematical calculations across the blackboard faster than anyone could follow her. The students couldn’t follow her train of thought.

The expression suggests a series of thoughts connected together in some logical progression (not randomly), in the same way that the cars of a train are linked together one after another.