Of horses: side by side, fastened together; figuratively, together.
In the midst of the working routine; at work.
Source: en.wiktionary.orgKalaiselvan said that when his father, Venkatachalam, a driver in the Tamil Nadu State Road Transport Corporation, died in harness some two decades back, it was his mother who brought them up and educated them amid struggles. Source: Internet
He started the race with twelve dogs and finished with eleven in harness. Source: Internet
Lincoln's appointments were designed to keep both the moderate and Radical factions in harness. Source: Internet
Most domesticated horses begin training under saddle or in harness between the ages of two and four. Source: Internet