It's a bandwagon in the literal sense, a wagon for transporting a circus or parade band – they had been around since the early 19th century. They only began to be used to attract attention at political rallies in the 1840s, whence came the expression "jumping on the bandwagon" to mean someone who attaches oneself to the excitement of a campaign without considering the issues.
This bandwagon is certainly built for the purpose; though it has the scrolls, bosses and scallops, it isn't the elaborate polychromed and gilded, over-the-top affair we usually associate with large commercial traveling circuses (certainly there is no ornamentation on the wheels to distinguish them from any common wagon). So this may be a local parade wagon, either for a July 4th celebration or a political rally. The uniforms are modestly trimmed – only two members have the gold braids we associate with circus attire. The second set of horses are somewhat smaller than the lead pair. The man at the reigns is merely the driver, so he doesn't get to toot his own horn, and he doesn't look like he really cares.
Many towns had bands for patriotic community events, fairs and entertainments (I have several photos that show bands at rallies for police and fire tournaments).
