Smaller cars and trucks. The tank car is also O gauge. As it stands now I can't run them.
Unfortunately this is one of the hazards of tinplate. The 600/2600 series cars have couplers 1/4" lower than the normally larger 800/2800 series cars. Where 2800 series cars line up well with a 263e, the 600 series lines up better with a 261e which is just a smaller shorter engine. I can't remember where it's at but there is a really good chart/explanation post around somewhere that makes things a bit clearer, hopefully someone else can post a link to it.
What I have found is that if you have car with the older style latch couplers instead of the box couplers there is usually enough wiggle to make it work. There are several different sizes within tinplate trains and some play together better than others.
I'd put together a conversion car with one of each height trucks/couplers so that I could enjoy my whole roster of cars
The latch couplers do help and are easier to bend. All my O gauge tinplate cars have the lower coupler height - and even those didn't match without a lot of adjustments. One of my main engines is a 263 with the high coupler. I didn't want to always use a conversion car, so I went to a LOT of trouble to lower the 263 coupler. I put up with it for a number of years, but I am really happy with the result and it still couples and uncouples properly. Not a modification for everyone. I would add pictures, but for some reason I can't upload them at the moment.