I'll say it the final time. It is normal for the non-tach motor to lag the tach controlled motor, it all depends on the specific friction encountered in the drive train. It's actually rare for both of them to start at the same time on a test block.
If you put a 10 ohm and 5 ohm resistor in parallel which resistor gets more current at a given voltage. There is a lot at play here. Once a motor starts turning and the engine moves the wheels on the non turning motor move but th worm gear doesn't. You now have some bind on the motor which means it needs even more current to start moving.
This is similar to what can happen on a Slaved diesel. Seems the E-8 are the most offending in this category. You have to clean both motor commutator and brushes with spray. Check the gear lash of the trucks, and lubricate properly to get them back in balance.
For the 2 PS-3 engines I saw this problem with the tach motor had a lower winding resistance then the non tach motor. You can measure and test all you want, in the end I put a replacement motor in for the tach motor and it was fixed. When the second engine came it I replace the tach motor first and it was fixed. G
I finally yanked the tach motor. Definitely not the same as the others I tested. Spin starts at lower voltage and it draws 200 mA at 2 volts. The obvious next step is to try a different motor set up for a PS3 tach reader. Do I have some PS3 engines that I would sacrifice to the tach reader motor? Yes. Do either of them have the right size worm gear? No. And from what I've read the gears are not removable. So for now I'm going to put the whole mess on the shelf and work on something more in line with my capability like lighting and scenery. 
200ma at two volts is normal for a Mabuchi RS-385 in good condition, I seriously doubt there's anything wrong with that motor. I have a whole gaggle of similar motors that have similar power consumption.
Well if it's not the motor it is certainly above my skill level. If both motors should have the same voltage the motors should both be turning at 2 volts. I can pick any motor and connect it in parallel to the tach motor and the tach motor is flying and the other motor won't budge. I'm also not seeing any delay with other engines between the motors. Both motors start turning as soon as the thumb wheel is rolled up to 1 or 2 smph. If the tach motor starts first then it's very hard to see. Regardless, the engine is broken. I will eventually try another tach motor just to be sure.
If you have a Mabuchi RS-385 that's not turning at all with 2VDC applied, I'd be looking closer at that motor. I just took six assorted Mabuchi RS-385 motors out of my parts box, all of them were spinning by the time my P/S got to 1.6 VDC, and humming along quite nicely at 2VDC. None of them were over 200ma power consumption, but they varied from around 130ma to 190ma. FWIW, I find that a shot of DeoxIT D5 on the commutator drops the current on most of them to lower values, one got to 110ma at 2.0 VDC. A drop of oil on the axle bearings frequently helps as well.
And it is NOT the motor. I connected another motor with a tach flywheel and was able to attach the tach reader in the right location. Powered up the engine and got the exact same result. It's very odd. If you touch the flywheel on the motor that won't spin you can feel that it is getting powered but it's like the motor is paralyzed. And it's not mechanical. I powered both motors off of a DC power supply and they ran together and turned the engine wheels very smoothly. And it's not the motors. I would try a different tach reader but cannot find where it plugs (if it does) under the giant pile of wires. Crap. I did try though. George and John, thanks for all the feedback.
I would get another 8 pin harness. Has power and motor leads. Swap it in and test again. I would guess a motor lead is damaged and can carry minor current and show continuity but not allow full current flow. I have seen grounded motor leads and I have seen that in steam harnesses, but not on a diesel motor lead. Maybe a full kink, or crush somewhere. G
I have my doubts that it's the tach reader. If the motor with the tach is not running away, it's getting the tach signal to the board.
I'll say again, the two motors when they're not on the track will very rarely run at the same speed or start up at the same time, this is totally normal.
You want a simple test to see if the second motor will pick up the slack?
Grab the tach flywheel lightly and put a load on it. As the current and voltage goes up to handle the load, the second motor will also receiver more power, and like magic, it'll be running.
I gave it the last rights for now. My best option for repair is to pull the PS3 set out of another engine. Probably the AS 616 I have that has always been a DCS bad actor. Then a sound file change. This has been a weird one for sure. I have had some early model PS2s up and die but have never had a PS3 fail on me like this. Quite a few hours of excellent performance and then this. 
Thanks for all the helpful advice. I sure know a lot more about Mabuchi DC motors!
Update to an old post. Turns out it was the motor. Maybe. I put the original motor back and powered both motors in parallel with a DC power supply. I was trying to see if there was maybe something wrong with the tach reader. At about 1.5 or so volts the tach wheel motor started but the other motor wouldn't budge until the voltage was more than 2 volts. I replaced that motor with one from another engine and both turned at the same voltage and the engine moved smoothly back and forth. I completely reassembled the engine and -- for now -- success! Runs like a dream.
