The OO garden railways I have read about are mostly very high tech, there was an American HO garden layout in Continental modeller recently, and I think it was also DCC. Electrics had to be spot on, so maybe not so good for Gn15 with small locos .
Maybe battery operated locos might be the answer, or a battery wired up to a loco, with battery in wagon. Simple 9v battery would probably do the job. Still have to build trackbed so that it is weatherproof and animal-proof.
It might be a good way to build a model of one of the longer miniature lines such as Ratty or Romney, using converted OO or HO locos. DCC ready ones would allow you to plug in a connection to a battery and keep isolated from track, by using only the motor connections on the DCC plug. With a tender loco it would be easy to put battery in the tender and still leave room for driver.
Narrow gauge Francophile interested in 1m, 60cm,50cm , 40cm and smaller gauges . Build in scales from 1/6th to 1/24th. Also 1/32nd and 1/35th using 16.5mm track to represent 50cm and 60cm gauges. http://www.rue-d-etropal.com