Have you looked into the LED boards made by Massoth for the LGB line of large scale trains. Not sure they can be AC powered, but they work well with DC power or like on my mostly dead rail garden layout, I use a 9vt battery to power each coach. Lasts for hours on a fully charged battery. Routing wires is just a matter of finding the most hidden route from the roof down a wall and thru the floor to get under the car to the wheels. A hot glue gun comes in handy(small hobby version is what I use) to hold wires in place. I have also used the metal foil tape for home AC duct work, but over time the stickyness breaks down, so I mostly use the hot glue gun. Not done any Marklin coaches as of yet, but several of my LGB coaches have Massoth LED boards and 9vt battery holders up in the roof.