NWSL 50 wheels, with a 0.050" tire width are intended for use on N fine scale trackage with a maximum flangeway width through switch frogs of 0.025" (NMRA RP-3 Finescale track), and ideally work better with a flangeway of 0.023" - 0.024". Normal N scale flangeway (NMRA standard S-3) is 0.033" max and a wheel machined to Finescale RP-4, which calls for a minimum tire width of 0.053", will drop into the wider flangeway at turnouts. NMRA Standard S-4 specifies a minimum tire width of 0.071". Wheels with a narrower tire than this, including NWSL 50, NWSL 64, Fox Valley (0.060" on my samples, except their wide tread (0.072")) and BLMA (0.056" on my samples) really require trackwork built to narrower flangeways than the maximum limit specified in NMRA Standard S-3. Narrower flangeways are not the whole story though as check gauge (running edge of one rail to outer edge of guard or wing rail) and span (distance between guard edges of wing and guard rails) also have to be compatible with check gauge (back of one wheel to gauge point of opposite wheel) and back-to-back dimensions of the wheels used.
Cheers,
