Would depend a lot on the size of the rail.
We used to have one that spanned 24 feet across the garage.
We used it to pull motors and stuff like that.
It was a medium size rail. 4inches tall I think.
Had one years later to span a bay in my workshop.
It was a full size rail (6inches tall) and spanned 14 feet.
After setting that monster I can tell you I won't do another one.
Those things are ungodly heavy.
But that 14 foot span would have supported 2000lbs easy.
The 24 foot span would deflect a bit with a motor and transmission hanging from in in the middle. But nothing unsafe.
You have to secure your rail on both ends. Not only from pulling inward during deflection but also from rolling over.
Bout the worst thing that could happen is to pull it down on yourself.
Be safe.
Pooh Bear