Sunset 3rdRail has opted to put single motor drives in their diesel models. I have quite a few and they ran great. I still have some, and have sold some models with 2 vertical motor drives. I personally do not like them. Mine never run as smoothly as the single motor drive models I have - even the old ones from the 80s. The most noticable thing is that they do not start and stop smoothly. One motor always seems to start or stop before the other, and the loco gives a jerk as the second catches up. The vertical motor drives have the gears along one side of the truck. On the examples I have, the truck sideframes stick out way too far. They are out as far as the sides of the body. On real locos, the trucks are tucked in underneath. Remember a real loco is about 10 feet wide and the track is only 4' 8 1/2 inches, so the wheels and sideframes are under the loco not out at the sides.
Another personal issue for me, is that the two motor units run way too fast. They seem to be more like slot cars than locos. I like my stuff to run slower, which the single motor drives with step down gearing do quite well. Two motor drives could probably be geared differently to run slower, but the market does not seem to care.
