MTH - Premier ABA Set Trailing Powered A unit seems to not want to work correctly in reverse.

I would check it as an AA unit without the B unit.  If the trailing A still does not work in reverse, it is likely a problem with the slave board.  It could be a pinched wire or an intermittent connection in the harness which will typically blow the motor FET in the slave board.

Thanks Jon, still did the same thing in the AA configuration so I will need to bring to my LHS and see if they can fix it as I couldn't find anything out of order after taking the shell off.

You might check to see if one of the motors in the trailing A unit came loose in its mount. That could make it run OK in one direction, but not in the other.

UPDATE - a few observations that it looks like a short. When I change directions so the Rear A unit is going in reverse it has all the correct lighting, but the number boards don't light up anymore (they light up fine in the default direction), but if I apply a little power it seems to make them glow a little bit and flicker and I'm pretty sure I hear a crackling sound. This happens along with the twitching and issues with the motor.

Thanks to Jon and Roy for your suggestions on getting this resolved.

I bet it is not electric, but rather mechanical.  Run it with Trail A shell off and watch flywheels.  It is not uncommon to have gear backlash bind a motor in reverse but not forward.  So watch the two slave unit motors.  One or both are binding.  Do not run this way for long, just to test.  Prolonged power with a motor locked up will burn up the motor, and or short the slave motor fet.  G