The train is worked upon at up to four levels at a time and fault reports are sent to the workshops in advance by the on-board computer system to minimize maintenance time. The ICE system is a polycentric network. Connections are offered in either 30-minute, hourly or bi-hourly intervals. Furthermore, additional services run during peak times, and some services call at lesser stations during off-peak times. Unlike the French TGV or the Japanese Shinkansen systems, the vehicles, tracks and operations were not designed as an integrated whole; rather, the ICE system has been integrated into Germany's pre-existing system of railway lines instead. One of the effects of this is that the ICE 3 trains can reach a speed of 300 km/h (186 mph) only on some stretches of line and cannot currently reach their maximum allowed speed of 330 km/h on German railway lines (though a speed of 320 km/h is reached by ICE 3 in France). The line most heavily utilised by ICE trains is the Mannheim–Frankfurt railway between Frankfurt and Mannheim due to the bundling of many ICE lines in that region. When considering all traffic (freight, local and long-distance passenger), the busiest line carrying ICE traffic is the Munich–Augsburg line, carrying about 300 trains per day. The network's main backbone consists of six north–south lines:   Furthermore, the network has two main east–west thoroughfares:  Some train lines extend past the core network and branch off to serve the following connections:  The "ICE Sprinter" trains are trains with fewer stops between Germany's major cities running in the morning and evening hours. They are tailored for business travellers or long-distance commuters and are marketed by DB as an alternative to domestic flights. Some of the Sprinter services continue as normal ICE services after reaching their destination. The service is usually half an hour faster than a standard ICE between the same cities. A reservation was mandatory on the ICE Sprinter until December 2015. The first Sprinter service was established between Munich and Frankfurt in 1992. Frankfurt-Hamburg followed in 1993 and Cologne-Hamburg in 1994. This service ran as a Metropolitan service between December 1996 and December 2004. In 1998, a Berlin-Frankfurt service was introduced and a service between Cologne and Stuttgart ran between December 2005 and October 2006. Until December 2006, a morning Sprinter service ran between Frankfurt and Munich (with an intermediate stop at Mannheim), taking 3:25 hours for the journey.