Lift-off

BD21 and CS454 offer factors to allow for possible "lift-off", where one axle of a multi-axle bogie (not vehicle) lifts off as a result of eg longitudinal geometry, transferring load to another axle.

BD21 suggests factors of 0.5/1.28 or 0.5/1.5. Both options are given for triple axle bogies. Is there any indication why two options are given or when either should be used? For 2 axle bogies, only 0.5/1.28 is given.

In the case of a normal bogie with load equally split between axles, applying factors of 0.5/1.28 loses 10% of the total load. It is not clear what the justification for this might be.

CS454 cleans things up a bit, giving factors of 0.5/1.5 throughout as the only option. It remains clear that these are factors to apply to bogies with equal axle loads. 

It seems unlikely that there are any situations where both lift-off and impact might apply. Impact implies speed; lift-off suggests situations, such as hump backed bridges, where slow movement is likely. 

Archie-M actually implements lift-off not by applying factors, but by halving the load on the axle set as lifted off, and shifting the missing load onto the other axle. This has an effect identical to applying 0.5/1.5 factors where the two axles involved start with equal weight. 

Lift-off is not applicable between axles that are not in the same bogie.