The Combine is probably my favourite of the Trainyard forms. I think if you watch it in action, you'll feel the same way:
That's right - with just a few squares, the Combine can neatly merge four (or three) in-phase cars, provided there's a two-square separation between each car. And because it gets reset after four cars pass through it, it can even take care of this eight-car surge:
(Note the use of the Phase Filter, which delays the red cars until the yellows have passed through the Combine.)
The Combine is really just a Loop used in combination with a Molehill, but it can be very useful, and it's such an elegant tool that you should probably use it whenever possible.
Next: The Phase Filter