First, I'm going to assume that you don't have the instruction sheet for your 44 tonner. If you do, follow along on the back, the exploded diagram. In the lower left corner, it shows the DCC decoder hookup diagram. I looked on Bachmann's website but couldn't find access to "instruction manuals".
Before you start, you'll need two items: your decoder (an NCE D15SR or Lenz LE1014W should work, I'm not familiar with Digitrax) and a low wattage (25 to 35 watt), fine tip soldering pencil. No, a "soldering iron" or "soldering gun" won't do. Once you see the size of the holes you need to solder, you'll understand.
Now, step by step, do the following:
1) Remove the body shell by removing the two screws in the "fuel tanks".
2) Place the chassis in front of you so that the writing on the PCB on top of the motor is right side up (white H800X-PCB01 SDIE A). I'll refer to the edge with that lettering as the "front". The other edge, with a green H800X in the right hand corner will be the "top".
3) Gently!, bend the two capacitors (the yellow components with two leads) towards you so they lay flat against the circuit board.
4) Remove the two very small brass clips on the left top edge. This will expose 7 holes along the edge. This is where you will connect the decoder.
5) Solder the leads from the decoder to the circuit board as follows: (you will need to refer to the decoder manual to tell you which color wire goes with which function)
a) Left most hole (#8) - power pickup right
b) Next hole to the right (#1) - motor positive (+)
c) Next " (#2) - reverse light
d) Next " (#4) - power pickup left
e) Next " (#5) - motor negative (-)
f) Next " (#6) - forward light
g) Next " (#7) - light common
Note 1: there is no hole #3.
Note 2: While soldering, be very careful not to create "solder bridges". That's were you use too much solder and it flows from one pad and bridges over to an adjacent pad.
Now you're hooked up. Put a piece of electrical tape on the bottom of the decoder and settle it on top of the PCB. Spongy double stick would work well. That done, it's time to test before you put the shell back on.
If this sounds beyond where you are now, or you don't have the necessary equipment, try to find a friendly, local modeler to help you.
Good luck and enjoy!