Plug-in and Train of thought

Plug-in noun - A printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities.

Train of thought and plug-in are semantically related. In some cases you can use "Train of thought" instead a noun phrase "Plug-in".

Train of thought noun - The connections that link the various parts of an event or argument together.

Plug-in and train of thought are semantically related. Sometimes you can use "Plug-in" instead a noun phrase "Train of thought".