The crossing is the track arrangement which ensures the intersection of two opposite running edges of a turnout or diamond crossing. It usually has one crossing vee and two wing rails (BS EN 13232-1:2003).

The crossing, in the common setup, has a gap section to allow the passage of the wheel flange on both directions. The safe running of the wheelset over this gap is ensured though the presence of the check rail on the outer rail and by the sufficiently wide wheel tread, which can pass over the gap and still keep a continuous contact area on the rails.

The tramway track crossing has a similar layout – with a crossing gap at the intersection of the two grooves of the crossing rails.

But, mainly due to the differences in wheel diameter, wheel profile and crossing angle, sometimes on tramway tracks the wheel tread support over the crossing gap cannot be efficiently provided. In such a case is allowed to have flange tip running over this short section of track. This is an acceptable approach because the tram is lighter compared to railway vehicles and also, usually,a low speed vehicle; the dynamic implications of the short term flange tip running are not considered significant and fully compensated by the benefits of using this solution.

In order to achieve this flange tip wheel running, the groove of the rail is filled to a certain level or, if special construction rail profiles are used, a shallow groove is manufactured. (NB: The construction rail is the rail used to manufacture switches and crossings, where the usage of a normal profile rail is not efficient).

The depth of the groove is designed in correlation with the wheel flange height and the groove wear tolerances. By using this approach the wheel runs over the crossing gap and the wheelset/bogie guidance is ensured by the groove of the opposite rail. Once the crossing gap is passed, the groove is gradually deepen back to its normal depth.

An ingenious application of this flange tip wheel running can be seen on the Manchester Metrolink lines, where, a crossover was installed (probably) to allow the diversion of the tram traffic. The setup does not presume frequent usage of the crossover and this sporadic usage of the diversion makes inefficient the usage of the normal crossing.
This arrangement does not have the crossing vee and the running edge of the main (though) line is continuous. When the siding (branch) route is approached by the vehicle, the crossing area is passed on the wheel flange tip, lifted and guided by the special arrangement visible in this picture:

This tramway crossing arrangement can be seen on the Metrolink tramway line installed on  Aytoun Street, Manchester. Here is a Google Street View of its location:

Note: The fact that this special arrangement is installed and visible at the above mentioned location does not mean it is not protected by industrial patents or intellectual property agreements. The author is sharing this only for information and has no intention to negatively affect the intellectual property rights potentially connected with this crossing setup.

References

BS EN 12232-1:2003. Railway applications – Track – Switches and crossings – Part 1: Definitions. British Standards Institution.