I've done this successfully a few times.
Depending on the age of the machines, they can be easy. The side rail nails are cheap, so I remove them with the full intent of damaging the old nails, not the side rails !
I use a couple of thin pieces of metal (30 cm or 40 cm ruler). First slide the piece of metal in the gap between the siderail and the body near the flipper button. GENTLY prize the side rail away from the body by twisting the piece of metal. DON'T prize the ruler AWAY from the body - you will bend the siderail - Just TWIST the ruler like the accelerator on a motor bike. The FIRST nail head should raise. I HOPE it has raised ! Now initially try an old pair of side cutters, and TWIST the nail counter clockwise - CAREFULLY. It MIGHT twist out - remember - they have a thread.
Continue along the entire siderail. But now you approach the nails from the bottom of the cab.
The aim is to not touch or bend the siderail.
Sometimes, the nails might be rusted, or just won't move out without more force. Be patient. Use a thin piece of metal NEXT to the nail, and a thin screwdriver and prize the nail out - but the golden rule is NOT to touch the siderail with ANYTHING sharp. Always place something there as protection.
I hope that sounds right !