He used very easily to lose his train of thought.

I lost my train of thought and the will to pull this one together.

She stopped, as though she'd lost her train of thought.

Jupiter had lost his train of thought for a moment.

I've completely lost my train of thought: what were we talking about?

He lost his train of thought in her smoldering eyes.

It wasn't so much that he'd lost his train of thought.

So whether different or similar- He stumbled on a stone, and lost his train of thought.

It was clear she had lost her train of thought.

But now was not the time to lose his train of thought.