In 1903, George Gibbs used his influence to contract with the Pennsylvania Railroad's shops in Altoona to build an all-steel prototype for the new subway. This car, numbered 3342, was tested in February 1904 and deemed to be too heavy for practical use in the new subway. It required further design changes before it could become serviceable. But most importantly, the all-steel prototype proved that an all-steel car could be feasible, and validated Gibbs' claims that the previously held fears of excessive vibration, poor insulation, and loud noise were unfounded. IRT engineers began modifying the all-steel design to lighten the cars to a more suitable weight. One of the largest breakthroughs came when engineers learned they could achieve a similar structural strength as the heavier car by constructing a "skeleton" floor frame made of thick, intersecting steel sills and crossmembers. This was in contrast to using a single thick, heavy sheet of steel for a large center sill that supported the car. Following this and other weight-reducing changes, the IRT was ready to go ahead with a production order of the new "Gibbs" cars, named after George Gibbs. Larger builders remained steadfast in their refusal to build all-steel cars. However, as a result of the generally successful introduction of the steel prototype car, the growing American Car & Foundry was willing to accept an order for steel cars. Three hundred were to be constructed, incorporating the latest modifications made by Gibbs and IRT engineers to reduce the weight of the cars. As it turned out, 200 of these cars were initially built, with an additional 100 as a supplementary order. The latter differed slightly from the base order by having aluminum trim on their roofs.