Build a R104 dual guage track: with these curved rails you can drive narrow gauge trains in between normal R104 curved tracks. To drive narrow gauge trains on either the outer side or the inner side, you only need B or C rails.
This is a set of 16 rails:
8x R104A rails for driving narrow gauge trains on the outer side.
8x R104D rails for driving narrow gauge trains on the inner side.
The letters A, B, C and D correspond to the studs as can be seen here: