Bachmann Freight Car Trucks  
This tip originally comes from George Schreyer's web site which I have expanded upon.  Many of us own some Bachmann rolling stock.  Though the scale is different, some of these cars still seem to fit in well with our 1940-1950 Aristo or USA freight cars.  The Bachmann cars are usually priced reasonably, are well made, and for the most part, painted nicely with crisp lettering and sharp graphics.  Ebay is a great source for these cars, as long as you don't go crazy over-bidding.  Most of these cars come with arch bar trucks, which sometimes require work before they will run well on your track.  This fix is fairly simple, and does not require special tools or materials.  I would recommend that you replace the plastic wheels with metal wheels since they will add weight and lower the center of gravity.  They roll better, and don't leave black gunk on the rails like plastic does. Bachmann makes some excellent metal wheels that are reasonably priced.
    Your first step is to remove the trucks, saving the attachment screws for re-installation.  Your wheels will first need to be re-gauged by pulling and twisting them.  Secure the wheel in it's new position with silicone or Goop.  It would be wise to invest in a Kadee wheel gauge to set the spacing more accurately.  Bachmann wheel sets will often catch the open point rail when not gauged properly.  Metal wheel sets also need to be re-gauged, as I have found them off right out of the package.
    Spin each axel to see if the wheels turn freely and there is no wobble.  If there is a wobble, discard the wheel set.  Lubricate the axel bearings with graphite or Teflon powder.  This is available from LaBelle and can be found at your local hobby shop.  A plastic compatible oil can be used for this purpose, but it will attract dirt in the outdoor environment and is not recommended.
    This next step is critical in doing your truck tune-up.  Place each truck on a flat level piece of straight track.  In many cases you will find that only three wheels touch the track.  Whoa !!   Major cause for derailments, guaranteeing that the car will be put on a siding or sold at a swap meet.  This twist in the truck frame can be easily fixed.  Heat the truck crossbeam for about 10 seconds with a 1200 Watt hair dryer.  Twist the truck firmly and hold it for a few seconds to let it set up in it's new position.  You may have to repeat this procedure several times until all wheels touch the track equally. 
    You can now re-install your trucks.  Check to be sure that the trucks move freely.  This procedure should allow you to run your Bachmann cars smoothly and derailment free.
     I have found that in some cases, LGB trucks have the same alignment problems.  Because they are made of a stiffer plastic, they can be fixed, but it will take more heat and time than the Bachmann trucks do.
    Now, for those who want to update their Bachmann freight cars to a more modern configuration, I have found that Aristo sprung trucks will fit these cars needing only one modification. A washer will need to be placed over the coupler pivot to shim the car higher so that the wheels do not hit the car under frame on curves.  I have done this with the three Bachmann hoppers that I bought and repainted, and am very happy with the results.