The weight of the rail in the crossing is to make a more stable base. By welding two lengths, you eliminate any joints in the crossing, cutting down on maintenance.
About 17 years ago, I installed an Omni crossing on a 2' railroad where we built an entirely new crossing. It was about 160' long crossing. The base was concrete with studs and clips to secure the rail. Then the Omni rubber inserts were installed.
The requirement by the design engineers for the crossing was to support the weight of very large firetrucks. The rail used was new 12#. The trick was the reinforced concrete track base.
The crossing is still in use and shows no significant wear.