I agree with the points made above. The joiners should be tight. If the layout is "permanent", soldering the joints is the best solution. Leave some gaps if you don't have good climate control (high heat will create expansion). Otherwise, make sure your joiners are tight - remove and squeeze them lightly with pliers to make the fit tighter. Adding more power feeders will also make a huge difference.
About the cars, do you mostly run them forward? If yes, then adding a bit of weight will make a difference. The best weights are sold in hobby stores, but you can glue your own, such as fishing weights with hot-gun glue.
If you do a lot of backing-up operations, then you might need more complex surgery on the cars for reliable operations. Removing the offending cars from the layout is the easiest solution. Otherwise, check the cars and see if the couplers are connected to the trucks (vs. the body of the car). Body-mounted couplers perform better when backing up. If you really want this, you will need to buy Kadee couplers, with coupler boxes, and install them on the body of the car (not the truck). You will need to clip off the coupler box from the truck, and install the coupler box on the underside of the car. This is better done with a 2-56 size screw. You will need a Kadee gauge to check the coupler height. Of course, this assumes that your cars have Kadee couplers (or something compatible). If you have horn hook couplers (google it!), then backing-up will be difficult whatever you do...
Simon