Here we have various track systems:
Scalextric: This is the standard Scalextric system (previously called Scalextric Sport) introducted around the year 2000. The stuff in this section fits most sets except odd sets called "Scalextric Start" and older pre-2000 sets that need an adaptor.
SCX: When available, this range is a rival to Scalextric that previously used the same pre-2000 series track but now uses it's own. It's not overly popular in the UK but is a technically better system.
Micro Scalextric: This is the Scalextric system with strong magnets and Matchbox sized cars. Very popular. Basically whatever Micro set you have, the items from this section fit - although the colours may be different as all add on track is black regardless of the original set. This includes the themed sets such as Simpsons, Looney Tunes, Batman etc.
Carrera: Carrera is Germany's leading slot system and one we really like. It's got good, rigid track and reliable electrics. It comes in two sizes - GO which is 1/43 and Evolution/Exclusiv which is 1/32 track and cars (1/24 cars with the rare Exclusiv system). Both sizes come as analogue and digital but use the same track within that size. If you have a battery set though, Carrera don't do extra track (but the battery sets are under £30 so should be considered a one-off toy anyway).