The answer seems to be rather straightforward. Trains are mostly kept inside the tracks through the conicity of the wheel, as apposed to trams, which are mostly kept inside by the flange. Flange rail interaction is more frictious and uncomfortable than managing the train containment between the rails through conicity. And more noisy. I've added an article.

you can also see these 

** The Relationship between Contact Angle Difference and Equivalent Conicity of High-speed EMU ( By: Zhang, Zhi Bo; Wu, Ping Bo; Luo, Ren; et al. )

**The Comparative Analysis of Calculating the Equivalent Conicity Methods in Vehicles of Mechanical Engineering ( By: Lin, Fengtao; Wang, Yue Ming; Dong, Xiaoqing))

Hi 

The conicity of the wheel keeps the axe (if the rail profile is in order) centered in the sense of the march. Also in the corners the conicitfy pushes the axe (and therefore the carriage) to the center of the corner. If the conicity is not in order ( indeed the wheel-rail equivalent conicity) the axe and thus the carriage makes a transversal movement and therefore a passager will feel a lateral movement on the ride.