Split the title onto multiple lines?

Examples for adding an abtract show the use of pipes | to break lines and include paragraphs. This works as well for the title and other yaml elements. For an abstract or title:

---
abstract: |
    What works for the abstract.

    Works for the title, too!
title: |
    | title 
    | subtitle
output: pdf_document 
---

For PDF output, experimentation revealed that the following works:

---
title:  'A title I want to  \nsplit on two lines'
author: 
date: 
output: pdf_document
---

That is two spaces followed by \n.

For an HTML output just us the <br> tag while if your output is a PDF or PDF presentation standard LaTeX code to break line given by \\ should work.

Example

---
title:  'A title I want to <br> split on two lines'
author: 
date: 
output:
  ioslides_presentation
---

For PDF

Just to rule out possibilities, I've tried to put \\ or \newline, both do not split, so for PDF seems to be a little bit tricky. \linebreak stop knitr parsing. Maybe another user can solve this question for knitr PDFs.