I agree that it is clear that for oversize luggage the 115" is "linear" where "linear" is defined as L + W + D. Except they also describe the example of a kayak that is ok as long as it is less than 115" long and I assume that they are not requiring W + D = 0 for the case L = 115".
I think it is safe to assume that a bicycle box with L + W + D = 115" or less is ok. But I still want to know if anyone has tried flying with an Amtrak bicycle box where L + W + D = 119.5"