Designation of multiple units as a locomotive

At what point does something become a locomotive? If I have two DMU's, but only one has an actual cab to operate from, are they both classified as locomotives and subject to the same tests and testing schedules? If the lack of an operating cab is not the difference, is it the presence of an engine? What if we say these are now EMU's: will the EMU without a cab be a locomotive? It would only have traction motors, no engine.

I'm sure the answer is probably murky at best, but surely there must be an answer since there's actual examples (except maybe a cabless DMU?).

any rail vehicle with propulsion or control equipment is a locomotive, be it EMU, DMU, cab car, or A or B unit Locomotive.

Moderator's Note: Moved to General Discussion: Locomotives, Rolling Stock, and Equipment.

DutchRailnut wrote:any rail vehicle with propulsion or control equipment is a locomotive, be it EMU, DMU, cab car, or A or B unit Locomotive.

So a coach in an Acela set is designated as a locomotive? (They do have traction motors, right?)

BostonUrbEx wrote:

DutchRailnut wrote:So a coach in an Acela set is designated as a locomotive? (They do have traction motors, right?)

Acela coaches are not motorized. Only the power units provide traction power.

In the case of married pair passenger MUs, are there FRA cab cards (ICC "blue forms") in both cabs or does a pair require only one cab card?

each MU car in married pair has its own blue card.