The main LEGO train systems are all roughly compatible. They all use a 6 stud gauge, so with a bit of fiddling, you can use all of the systems together. If you want to be most compatible with the current track offerings, you’re better off with 9V and RC/PF track.
What gauge is Lego train track?
The train system is sometimes referred to as ‘L- gauge ‘ among fans, in reference to traditional model railway scales. Lego trains use a nominal gauge of 37. 5mm, 5. 5mm wider than O gauge, derived from a centreline gauge of 40mm (five lego studs).