I've never heard the term "Train Garden" here in Washington DC/Suburbs/Montgomery County area. I assume it is a Baltimore term...?
Regardless, there are some great displays to be head at the US Botanic Gardens, Nat. Christmas Tree, Brookside Gardens (Wheaton, MD including a new display of the Georgetown to Glen Echo Trolley complete with exact replica of GE Amusement Park, Neon Sign, Stone tower, Carousel, etc), and more. See the link below for info on Walkersville Southern's Santa Train and National Capital Tracker displays.