Inline loop

This little junction is good for low density networks with trains that are short enough to fit inside the loop or so heavy that they cannot scale even a single slope.

U-Turn Junction

Pros:

• Cheap & small
• No sharp turns

Cons:

• Longer trains can block the mainline when in the turning lane

Flyover loop

With the help of a bridge (or a tunnel) the U-turn loop can be made as wide as needed, without requiring any modifications to the mainline.

U-Turn Junction

Design tips

• Make sure that the turning spur is long enough that the longest train can fit completely inside it. Otherwise a train that took the turn will block the trains that go straight.
• Make sure to place a path signal in front of the track split. Otherwise a turning train will block the straight direction even if it doesn't occupy the actual track.
• There is no need for excessively wide turns, because the train that took the turn is likely to stop anyway when merging into the mainline.
• A bridge works slightly better than a tunnel, because its downward slope is located closer to the exit, and thus it will speed up the exiting train a bit longer.
• As with any sideline merge, it is a good idea to install a priority.