The point is that all human life if of equally value, morally it is wrong to sacrifice one life for another. If the hypothetical city has 100,000 habitants then that baby's life has been weighed 100,000 times and been deemed of less worth than that of another human being.
That is what makes the sacrifice so very wrong.
Your hypothetical motorcycylist is somewhat different, as you have been placed in a situation where you have to make an immidiate decision, where the cyclist is already going to die on the bridge, and without an antagonising force. Running over him is still morally wrong, but perhaps easier to forgive.
Of course, in that case you could always try to get out and move the cyclist, or get everyone off the bus and tell them to run. Both are better choices than running him over.
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