On the whole, yes, a new Scalextric car can run on the old classic Scalextric track. The dimensions of all the interfaces are the same. However, there are some exceptions. The modern cars have fixed front axles which mean they like relatively flat track otherwise a front wheel will lift the guide up causing the pick up braids to come away from the track rails and the car can stop or hesitate.
Classic Scalextric track features to avoid are the traditional banked
corners and the hump back bridge as some new cars belly out on that.
There may also be a problem negotiating the rock used in the classic
The other way around, can the classic Scalextric cars be used on the modern Scalextric Sport track?
This now does depend on the cars and in particular the guide used.
Some of the older Scalextric cars fitted with a pin guide can have
problems with the cross over pieces and junctions on Digital track.
Modern Scalextric Sport track offers less mechanical grip compared to
the classic Scalextric track and so grip can be a problem. This can be
overcome by using Max Grip tyres or adding a Neodymium magnet fitted between the motor and rear axle.
Another great insight from Scalextric Car Restorations