Most brands of model engines as opposed to toy train set diesels are fine. Generally it will be a quality unit if the motor is mounted fore & aft in the middle of the chassis driving the wheels through worm gears and shafts. The toy train engines have an "East-West" motor that is part of one truck and the other truck is a dummy. Most of the plastic diesels such as Lifelike Proto 2000 (frequently refered to as P2K), all Stewarts, Kato, Bachmann Spectrum are using a copy of the Athearn drive. Be careful of Bachmann Spectrum 6 axle diesels. The trucks are on a kind of ball joint and they have a tendency to derail easily in curves if the track is not absolutely perfect. Before all those who swear by Kato, Atlas, or whatever their favorite brand of locomotive is jump on me, I said the drive is a copy of Athearn. These other brands hard wire their connections instead of using sliders, and they have can motors as standard equipment so they will run better out of the box than stock blue box Athearn. There are a few things to check however on all locomotives. Buy a NMRA standards gauge and use it to check that the track is in gauge and that locomotive wheel sets are also in gauge. Athearn is famous for having one or more wheel sets too narrow out of the box. If you hold the locomotive upside down and gently rock the wheels on an axle in opposite direction while pulling out or pushing in as needed, the wheels will move on the plastic gear until they are in gauge. Atlas track can also be the problem. The flex track is not bad, and even snap track is not bad except that buying 9" sections to do an entire layout is expensive plus it has potential for a lot of bad electrical connections. The problem with Atlas is the turnouts. The points are made of sheet metal riveted in place. They have a tendencey to loosen up over time and the points lay over increasing the spacing between the tracks right through the points. I repaired some Atlas snap switches by taking a piece of rail, cutting it about 1/2" shorter than the points, filing it to fit and gluing it carefully to the back side of the points with a little epoxy to reinforce the points. I came to the conclusion that the process was just too tedious. I have Shinohara turnouts now, but am not sure how much of an improvement they are. I'm thinking on my next layout to handlay all turnouts so the points will each be one rail all the way to the frog, with gaps cut near the frog to avoid shorts as needed. Other things to watch out for are bumps and dips in the curves on either rail. If one Rail dips in a curve, but the other stays level, the wheels will tend to step off the track at the dip. Turnouts that are not flat will also cause problems. A turnout on a grade will work as long as the gradient doesn't change through the entire turnout, but if the turnout is put into any sort of twist, it will derail trains. I realise this response is a bit long, but I hope it helps. There is probably something else that I forgot to mention, but someone else will contribute anything I missed.