K-Line Big Boy 4015S - doesn't respond to throttle (legacy/command or conventional)

Well, I actually see two boards unless you're counting the motherboard.  All of the boards are bog standard Lionel boards, easy to get. 

First step is to make SURE that all the connections to the DCDR are good and you actually have power to the DCDR.  The red arrow points to the power and motor connection, make sure you have track power, ground, and good connections to the motors.

The blue arrow points to the PWM inputs from the R2LC, make sure that cable is good and you have continuity for all the pins.

Next, unplug the black power/motor plug from the DCDR.  Connect a DC power supply (between around 3V and 6V) do one of the motors, and have the wheels suspended or on rollers.  If both of the motors don't run, that's the first thing to sort out.  If they do, then we move on.

This board is an R2LC-C08, you can try swapping it with another TMCC locomotive to see if that helps.  If not, then the DCDR is likely the suspect.

If you need a DCDR, drop me a line to my profile address, I have a bunch of them.  From the description of stalling the motors, you could have cooked the triacs on the DCDR.

Thanks for your detailed reply, John. I was definitely counting the motherboard as the third board 

Connectors to the DCDR look ok. I measured 17+ VAC across the black/red pins of the black connector with track powered up, and both motors turned when I connected 3V DC (2x AA batteries) to the brown/gray pins. I ohm'd out the four wires on the PWM connector from the R2LC and got a beep and very small resistance reading for each.

I swapped the R2LC-C08 board with one from another locomotive and the behavior was exactly the same - lights, sound, direction, couplers all worked but no motion.

It seems the DCDR board is to blame. What typically "cooks the triacs" on a DCDR?

Would it be better to get an ERR Cruise Commander board instead of another Lionel DCDR? Are there any downsides I should consider? I love the idea of adding cruise and smoother low speed to my locomotives. Looks like the Cruise Commander M would fit the Big Boy.

If you want to add cruise, for sure the CC-M is the way to go, it's close to a drop-in replacement.  You just have to run one wire from the CC-M to pin 24 of the R2LC socket.

What kills the triacs are usually an overload, but it does take quite a bit to do the job.  I'd check all the wiring closely for any pinched wires that might short to the frame.