The door mechanism is like a fine clock or watch, well made with brass bushings on the ends of the drive shafts. The reason the doors stick is lack of lubrication. The door rack is sprung to allow for a slight over travel when opening or closing. This is were the issue occurs. When the over travel happens the springs on the slide shoes compress this protects the gear box. When the doors go to close the pressure on the rack will bind the door causing the dreaded blinking lights. simple lube will stop this 99% of the time. Now when you remove the two center door units (attached to the motor/transmission) underneath each screw is a felt washer this is important as a spacer.
Another issue came up with one set of doors happened recently. The doors all opened but one set did not close. What I found is the dog bone shaft way key way had either opened just a tad or moved back on the shaft. Either way this is made of hard plastic and I would not recommend messing with it to either move or try to close. My solution was to cut a thin strip of 3M electrical tape and with the unit out of the car and the dog bone installed. Rap it around the part of the shaft way/key way that is slotted. The only thing that this is stopping is the dog bone from dropping out. Should you have any questions please just contact me via message.
The motor truck on my R-27 came apart, what happened is the brass gear split. So I order a new one from Lionel (yup they have them) removed the old one filled with grease and back in service. Be careful as the motor mounting plate is very easily stripped ( yup again I have that tee shirt) ordered from Lionel and replaced.