Easy ... swap the wire feeds to the board

Easy, the wire feeds to the board in one of the locos is backwards. Swap the wires to the board and the loco will run the other direction.

Which loco do you switch?

If these are your only two locos, it doesn't much matter on which one you change the wires.

If you have other locos, check whether the A unit goes forward when the other locos do. If so, change the B unit. If the A unit runs counter to the other locos, change its wiring.

Reason for this: everybody should go the same direction for the setting on the power pack or cab.

If you have no other locos but expect to acquire some, then depend on the convention: for a loco in motion the positive DC rail is to the right of an engineer facing the direction of travel.   If the output posts of your power pack aren't labeled + / -, then you need a voltmeter. Or you can just wait until you get the next loco, and change wires if that loco runs counter to your AB set.

DrJolS

Botchmann Wire Colors and Motor Orientation

Several years ago I was asked to take a look at my Nephew's Bachmann F7 AB set which, out of the box, was exhibiting the same problem you are describing. The first thing I discovered was that, although they had used wires with the correct colors, they had not wired the locomotives using the NMRA color scheme (See: S-9.1.1 Electrical Interface & Wire Color Code) and color coding was inconsistent between the two units.

From S-9.1.1 Section B.

RED from right-hand rail power pick-up ... to motor or interface

ORANGE from interface to motor brush (+) connected to right-hand rail …

BLACK from left-hand rail power pick-up to motor or interface

GRAY from interface to motor brush (-) connected to left-hand rail

After correcting the wiring the locomotives still ran in opposite directions? With a little more debugging I discovered that Bachmann had also rotated the orientation of the motor in the B unit 180 degrees effectively swapping the + and - motor terminals. After swapping the motor wiring the loc's were finally running in the same direction. The wiring could not have been sorted out without a Volt meter.

An Option

I personally like to sort out the wiring; however, you could wire the locomotives up for DCC the way they are and then correct the direction power by changing the CV29, Normal Direction of Travel (NDOT) bit

The Normal Direction of Travel, or NDOT for short, lets you set up your locos to run either direction.  The decoder determines which way the loco will move independent of track polarity, you can set up either direction as forward depending on the prototype.

Art

Typical Bachmann - Poor QC

While I like Bachmann locomotives in general, the wiring is the one place that many of them fall short in my experience.  I make it a habit to completely rewire any Bachmann which I have to open for any reason.  There is simply no excuse for having two DC locomotives which run in different directions from the factory - it's poor quality control.

Don't be afraid to rewire them, or at least to swap the motor leads.  It's an easy job, and will solve the problem.

Befor you rewire

You said you had taken them apart.

You might check to see if it is possible to put the body shell on the B unit backwards.  Most only go on one way, but you never know.....

Non-issue

That shouldn't matter.  The chassis should run the correct direction even without the shell in place.  It may or may not look right, but on DC the track polarity is the only thing which is supposed to affect the direction of travel.

Sorry - I was thinking DCC

I had to go back and re-read the original post. Sorry - I was thinking DCC, not DC. gs