Hey Monty,
I had to locate a breakdown unit before I could give you a proper answer. First, Marty is right, the Breakdown B units were pre-LEGACY. The "BDU" keypad layout on the CAB2 has custom icons for this engine--but you can get all the same functionality using a CAB1, too.
The game starts with an AUX1-Boost (CAB1) or by pressing the "broken wrench" icon on CAB2. You'll hear "she's running smooth..." and the game is underway. Once the game starts, the prime mover's condition will slowly deteriorate. If you ignore the problem and keep rolling, total engine failure will eventually result. The way you drive the train will determine how quickly or slowly the engine dies. I won't say more, since that's part of the fun.
You can also avoid engine failure by stopping where you are and shutting the engine down with a numeric <5> key (CAB1) or the "First Aid Kit with a slash" icon (CAB2). In this case, you'll hear some dialog about an unscheduled stop and the dispatcher will note your block is flagged.
If you can keep the engine alive long enough to get back to the yard, shut it down with an AUX1,<5> (CAB1) or the "First Aid Kit" icon. In this case, the dialog will reflect that you made it back in one piece, but that booster definitely needs some service. There are a number of different pathways through the game scenario; these are reflected by different dialog.
As to why key operation seemed inconsistent: The tower com and crew talk (aka <7> and <2> do not trigger dialog on this product--only starting the game makes it talk. And the first aid or <5> key buttons only start playing dialog once the crew notices there's something wrong, so they might be keys you noticed worked sometimes but not others. The other keypad keys work all the time, as you'd expect: volume, RPM ramp up/down and smoke on/off.
Hope that helps. Thanks for asking--you gave me an excuse to play with trains this afternoon!
Cheers,
Rudy Trubitt
Director of Audio
Lionel LLC