Bibliophage has it mostly right, but I do want to poin out a couple of inaccuracies.
In North American parlance, a locomotive with a prime mover and no cab is a B unit, not a drone. Slugs may or may not have cabs. CSX runs phosphate trains in Florida with cabbed slugs; Conrail used cabless slugs to switch their major yards. A calf is a B unit of a low carbody switch engine
Matched pairs was a common practice until the early '60s, especially with passenger power. Today, when motive power can and does wander the country in power sharing agreements, the practice is almost unheard of. That doesn’t mean that a consist can’t remain intact for weeks at a time, however.
Model railroader and railfan since I was in single digits…