I'd like to know if I can put lists on the same vertical line in Trello. I couldn't do it but it would be pretty good for my work. Some lists don't need to be separated in different columns.
You don't mention what platform or browser, but the article Building trello.com for multiple devices describes how Trello is laid out, and so how you can control the layout.
So, if you create a narrow browser window, and/or increase the font size, it should eventually switch to a single column view.
Here is a series of screenshots, showing Trello in gradually narrower and narrower windows, showing how it rearranges its contents to fit the space available.