The set extension program is a system with continuation with which you can extend the track sections of the starter packs in stages. After starting with the 81530/81535 or 81562 starter set, you can expand with the E 8190 or E 8191 set. Then you can systematically expand your layout in any order with track packs T1 8192, T2 8193 and T3 8194. If you start with the 81781/81785 or 81852 starter sets, you already have the E 8191 expansion set integrated and further expansion can then in turn be easily done with the T1 8192, T2 8193 and T3 8194 track sets. The 8198 catenary packs for S+E and 8199 for T1+T2+T3 make it easier for the set program to get started with functionally true catenary operation, with which two trains can be controlled independently of each other on one track.