[gargravarr]

In a sleeping capsule, passenger's ranges of movement up/down and forward/backward would inherently be more restricted than in a seat. If necessary, a belt could be provided, although it might not be as comfortable because there's no guarantee on where the person's waist will be when lying down.

The way I picture it is replacing seats as they are currently laid out in e.g. a 747 - 3 seats become stacked capsules -> aisle -> 3/4 seats become stacked capsules -> aisle -> 3 seats become stacked capsule. I don't deny that getting into these capsules would be awkward for people with limited mobility, but I do imagine it is within the realms of possibility while meeting safety regulations.