933-2901 3 Stall Modern Roundhouse Extension Kit16 x 20 1 8 x 5 1 2" 40. 6 x 51. 1 x 14cm The Walthers Cornerstone 3 Stall Modern Roundhouse Addition enlarges the hallmark of steam era railroading on your HO layout the 3 Stall Modern Roundhouse. If you model the diesel era you'll also find this kit to be a centerpiece of your servicing facility since some of these classics are still used today for servicing diesels. No matter the weather, workers could easily inspect and service locos under cover.