Complex sentence: Until the start of Silver Line service, excess Rosslyn Tunnel capacity was used by additional Orange Line trains that traveled along the Blue Line to Largo (as opposed to the normal Orange Line terminus at New Carrollton).
Split into simpler sentences: Until the start of Silver Line Service, excess Rosslyn Tunnel capacity will be used by additional Orange Line trains. The additional Orange Line trains will travel along the Blue Line to Largo (as opposed to the normal Orange Line terminus at New Carrollton).