Oh, no, no, we were just doing a little model railroading.

4 seconds sound clip from the Back to the Future Part III movie soundboard.

You can hear this line at 00:49:07.828 in the DVD version of the movie.

Quote context

[...]

- Pull up to the DeLorean! Pushing the DeLorean up to 88 miles per hour!

- It couldn't be simpler.

- It's Clara. Quick, cover the DeLorean!

- Hello.

- Why, hello. This is quite a surprise.

- Well, I hope I'm not disturbing anything.

- Oh, no, no, we were just doing a little model railroading.

- Emmett, when my bags were thrown from the wagon, my telescope was damaged. And since you mentioned an interest in science, I thought you might be able to repair it for me.

- I would pay you, of course.

- Oh, no, no, no. I wouldn't think of charging you for this.

- Well, let's have a look at it.

- I think a lens may be out of alignment, because if you move it this way, the image turns fuzzy. See? But if you turn it the other way then...

- Everything becomes... clear.

[...]