(RECORDED IN CARSON CITY, NV)
This was the first type of signal to be invented. In the early days switchmen (people who would stay by crossings) would walk out in the middle of the street with a red lantern swinging to warn drivers of a train coming by. To prevent a lot of hiring to people, the wigwag signal was invented to simulate a swinging lantern.
These came around the late 1910s. Their unreliability to swing proved to be their downfall, and only about 1% of all wigwags exist today, and even those are disappearing rapidly. There are less than 100 wigwags remaining in service Today.
There are many different types of wigwag signals that were made, and while I won't cover any of them, Dan's Wigwag Site has lots more information and diagrams on just about all wigwags that were made.
In an unrelated note, the crossbuck has been around since 1883!