Great!

“In use” is perfect here. I’m in the US for a few months and just joined a gym near my house. Now, you can reserve a lane on your phone! I had no idea some places did that.

Haaaaaaaaa oh man. They are mostly gone in the US, at least as far as I can tell. The only time people fax anymore is via computers. A lot of copy machines can scan a document and fax it. And a lot of companies have fax lines set up to receive faxes, scan them automatically, and process them. I guarantee you the fax number at my bank was not printing a hard copy of my signed form – it was computerized. But the bank never got around to converting that to e-mail, so here we are.

Yes, this is right, except that “cashier” is a person, at least in American English. You could say, “…because only one checkout line was in use.”