Come/spring to mind = (of a thought) occur to someone.

• The idea of global warming comes to mind when we see what is happening.
• A : Have you any suggestion?
• B: Nothing springs to mind, I am afraid.
• A: Just say whatever comes to mind, it doesn’t matter.
• Say the word “Australia” and a vision of beaches and blue seas immediately springs to mind.

A train of thought = someone’s pattern of thinking or sequence of ideas; what one was just thinking about.

• When the phone rang, it derailed my train of thought.  I totally forgot what I was talking about.
• Please don’t interrupt my train of thought when I am writing.
• My train of thought is probably not as clear as it should be.

The shape of the things to come = an indication of what is likely to become popular in the future.

• E-commerce seems to be the shape of things to come.
• “The shape of the things to come” is a work of science fiction by H.G.Wells, published in 1933, which speculates on future events from 1933 until the year 2106.