Lionel 8" boxcars?

I'm assuming you're referring to the 6014-based, "scout" type double-door boxcars originating from the postwar period.   The modern-era versions harking back to MPC are very light due primarily to replacing the stamped-steel frame with a plastic one.  You can easily swap them out with a postwar frame since the slot and screw mounting holes still exist on the modern-era molds, or you can just add some weight on the inside of the plastic frame and you'll get good results that way as well (the doors are molded in so you won't be able to see the weights unless you pop the shell off anyway).

Yes, the MPC version is light, as is the newer LTI versions. BUT weight is NOT the issue.

People can choose to argue we me if they wish, but I now have ZERO derailments other than my own error... for going on 20 years of success! And that is pushing backwards 12 car plus trains (though 027 curves with "S" configurations... a derailment invitation) with a couple short light short MPC cars directly behind the engine, pushing much heavier cars with die-cast trucks.

The trouble with derailments is much more the loose rivet that holds the truck in place to the body. Wiggle a truck and see what I mean. Because of all that play, especially when pushing a train backwards, with all the pressure being on the couplers, the trucks will roll up over the rails, especially on  027 curves and switch tracks.

I drill out the rivets and replace them with a 8/32 truss screw and a lock nut. I tighten it all the way, then loosen it just enough so the truck swivels left to right. In the case of visibility, like a flat car or gondola, I either paint the screw head, or use a pre-blackened hex screw. On die-cast trucks with screws going into the truck, I borrowed a friend's idea, tightening those up too, as some of them have some substantial wiggle in them.

You could replace the plastic trucks with die-cast ones, but I have found IT IS NOT necessary... unless you like the look of them. Buying used postwar trucks can be cheaper than buying brand new ones.

Just for the record, other causes of derailments outside of track, are couplers not having sufficient room inside the closed knuckle for movement around curves. More common with rolling stock from different manufacturers. Though flashing on the plastic trucks will cause it too.

The other reason, especially buying used cars, is the wheels being slightly out of gauge on the same truck. I'll swap wheel sets around to make sure the wheel sets on one truck are lined up with each other.

I have NEVER found it necessary to add weight to rolling stock outside of cars with center rail pick ups or gear-driven operating cars.

And as far as improving rolling stock, I do a lot of repainting... it's amazing how much better the short Lionel, K-Line and MARX cars can look with small added details and good decaling.

I save my weights for adding to locomotives, which helps lighter locos with traction. One main reason I run my layout with the option for DC current. The smaller less expensive Lionel engines without the circuit board e-unit allows for additional weight. My little Lionel Industrial Switchers can pull 15 cars with ease.

When I go to train shows, I often spend time looking through the junk boxes that some vendors bring. I often find postwar 6014's with busted up shells, but perfectly good trucks, couplers, and frames. I often get them for a couple dollars. I remove the shell, give the trucks, couplers, and frames a good cleaning, and mount a clean MPC era shell on them. That works good for me.

Another step the Scout type cars, beat-up cars, are good for is as trackside storage and other buildings, as used all over the place.  I have quite a few Scout boxcars in great condition in my collection, some more uncommon than most, different variations.  Those, naturally, are to remain as is.  But, the bargains found in boxes at shows and garage sales/flea markets can be well put to use.

Jesse    TCA