From OpenTTD

This type of junction is only good for lines where you want trains to go one direction only, from a raw material industry to a production industry or a final industry, for example from a Iron Ore Mine to a Steel Mill or from a Coal Mine to a Power Station. Note that a tunnel was used instead of a bridge. This is because tunnels have no maximum speed. Speed is everything so if you have to build a bridge, build the fastest one you can.

Pros:

• Cheap and easy to build.
• Can handle two trains at a time. (Several without great impact on the main track speed, if built correctly)
• No crossovers - replaced with a tunnel.

Cons:

• Trains must slow down due to a steep curve. (can be fixed by making the curve loop slightly further)
• Trains go to one direction only
• Trains must turn left to go right
• Tunnel increases minimum train spacing. (Can be fixed by using two or more tunnels)

"Pictures & Rail design tips written by Owen Rudge - please visit his site for more information."

[edit] Second Version

This version is shown here as alternative, but it is by all means worse. In this version mainlines have tunnels where you cannot place signals.