Watching out the train window as some workmen positioned and riveted the steel beams of an electrical power-line tower, Gilbert decided to create a children's construction kit: not just a toy, but an assemblage of metal beams with evenly spaced holes for bolts to pass through, screws, bolts, pulleys, gears and eventually even engines. A British toy company called Meccano Company was then selling a similar kit, but Gilbert's Erector set was more realistic and had a number of technical advantages --- most notably, steel beams that were not flat but bent lengthwise at a 90-degree angle, so that four of them nested side-to-side formed a very sturdy, square, hollow support beam.

Gilbert began selling the "Mysto Erector Structural Steel Builder" in 1913, backed by the first major American ad campaign for a toy. The Erector set quickly became one of the most popular toys of all time: living rooms across the country were transformed into miniature metropoles, filled with skyscrapers, bridges and railways. Those kids who already owned a set would beg Santa annually for an upgrade, aiming for the elusive "No. 12 1/2" deluxe kit that came with blueprints for the "Mysterious Walking Giant" robot. It is difficult for anyone under the age of 35 today to appreciate just how popular the Erector set was for over half a century.